diff --git a/data/maps/sample_arena.yaml b/data/maps/sample_arena.yaml
new file mode 100644
index 0000000..860eb4e
--- /dev/null
+++ b/data/maps/sample_arena.yaml
@@ -0,0 +1,48 @@
+name: "Arène d'essai"
+square_size_ft: 5
+
+terrain: |
+  ####################
+  #......######......#
+  #..C...######...C..#
+  #......~..~........#
+  #..TT..~..~...TT...#
+  #......~..~........#
+  #..C............C..#
+  ####################
+
+legend:
+  "#": { type: wall, move_cost: null, blocks_los: true }
+  ".": { type: floor, move_cost: 1 }
+  "T": { type: rubble, move_cost: 2 }
+  "~": { type: water, move_cost: 2 }
+  "C": { type: pillar, move_cost: null, cover: true }
+
+elevation: |
+  ....................
+  ..1111..............
+  ..1111..............
+  ....................
+  ....................
+  ....................
+  ....................
+  ....................
+
+zones: |
+  ....................
+  .AAAA..........BBBB.
+  .AAAA..........BBBB.
+  ....................
+  ....................
+  ....................
+  ....................
+  ....................
+
+markers:
+  autel: [1, 16]
+  brasier_ouest: [6, 5]
+  brasier_est: [6, 14]
+
+deployment:
+  players: A
+  monsters: B
diff --git a/src/pf1e_simulator/grid.py b/src/pf1e_simulator/grid.py
new file mode 100644
index 0000000..a37eb4e
--- /dev/null
+++ b/src/pf1e_simulator/grid.py
@@ -0,0 +1,123 @@
+"""Square grid: PF1e distances, movement costs, and Dijkstra pathing.
+
+Diagonal movement alternates 1-2-1-2 squares (the 5-10-5-10 ft rule), so the
+Dijkstra state carries the parity of diagonals used. A diagonal step into
+difficult terrain costs a flat 3 squares and still counts as a diagonal for
+the alternation (table convention). A diagonal step requires both flanking
+orthogonal squares to be terrain-passable (no cutting wall corners); creature
+occupancy never blocks a diagonal, and occupied squares cannot be entered in
+Phase 0 (moving through allies is a later refinement).
+"""
+
+from __future__ import annotations
+
+import heapq
+from typing import TYPE_CHECKING
+
+if TYPE_CHECKING:
+    from pf1e_simulator.map import MapSpec, Pos, TerrainType
+
+_DIFFICULT_TERRAIN_COST = 2  # rubble, water, etc. count as 2 squares (PF1e)
+
+_DIRECTIONS: tuple[Pos, ...] = (
+    (-1, -1),
+    (-1, 0),
+    (-1, 1),
+    (0, -1),
+    (0, 1),
+    (1, -1),
+    (1, 0),
+    (1, 1),
+)
+
+
+class Grid:
+    """Terrain view of a MapSpec with movement and distance queries."""
+
+    def __init__(self, cells: tuple[tuple[TerrainType, ...], ...]) -> None:
+        self._cells = cells
+        self._height = len(cells)
+        self._width = len(cells[0]) if cells else 0
+
+    @classmethod
+    def from_spec(cls, spec: MapSpec) -> Grid:
+        cells = tuple(tuple(spec.legend[ch] for ch in row) for row in spec.terrain)
+        return cls(cells)
+
+    @property
+    def height(self) -> int:
+        return self._height
+
+    @property
+    def width(self) -> int:
+        return self._width
+
+    def in_bounds(self, pos: Pos) -> bool:
+        row, col = pos
+        return 0 <= row < self._height and 0 <= col < self._width
+
+    def terrain_at(self, pos: Pos) -> TerrainType:
+        row, col = pos
+        return self._cells[row][col]
+
+    def passable(self, pos: Pos) -> bool:
+        return self.in_bounds(pos) and self.terrain_at(pos).move_cost is not None
+
+    def distance(self, a: Pos, b: Pos) -> int:
+        """Distance in squares under the 5-10-5-10 diagonal rule."""
+        d_row = abs(a[0] - b[0])
+        d_col = abs(a[1] - b[1])
+        return max(d_row, d_col) + min(d_row, d_col) // 2
+
+    def step_cost(self, frm: Pos, to: Pos, diagonals_used: int) -> int:
+        """Cost in squares of one step; diagonals_used is the count so far."""
+        cost = self.terrain_at(to).move_cost
+        if cost is None:
+            msg = f"cannot step into impassable square {to}"
+            raise ValueError(msg)
+        is_diagonal = frm[0] != to[0] and frm[1] != to[1]
+        if not is_diagonal:
+            return cost
+        if cost >= _DIFFICULT_TERRAIN_COST:
+            return 3
+        return 1 + (diagonals_used % 2)
+
+    def diagonal_allowed(self, frm: Pos, to: Pos) -> bool:
+        flank_a = (frm[0], to[1])
+        flank_b = (to[0], frm[1])
+        return self.passable(flank_a) and self.passable(flank_b)
+
+    def reachable(self, start: Pos, budget: int, blocked: frozenset[Pos]) -> dict[Pos, int]:
+        """Cheapest movement cost per reachable square, capped at budget.
+
+        `blocked` holds creature-occupied squares: unenterable, but never
+        affecting diagonal_allowed. The start square is included at cost 0.
+        """
+        best_per_state: dict[tuple[Pos, int], int] = {(start, 0): 0}
+        results: dict[Pos, int] = {start: 0}
+        heap: list[tuple[int, Pos, int]] = [(0, start, 0)]
+        while heap:
+            cost, pos, parity = heapq.heappop(heap)
+            if cost > best_per_state.get((pos, parity), cost):
+                continue
+            row, col = pos
+            for d_row, d_col in _DIRECTIONS:
+                nxt = (row + d_row, col + d_col)
+                if nxt == start or nxt in blocked or not self.passable(nxt):
+                    continue
+                is_diagonal = d_row != 0 and d_col != 0
+                if is_diagonal and not self.diagonal_allowed(pos, nxt):
+                    continue
+                new_cost = cost + self.step_cost(pos, nxt, parity)
+                if new_cost > budget:
+                    continue
+                new_parity = (parity + 1) % 2 if is_diagonal else parity
+                state = (nxt, new_parity)
+                known = best_per_state.get(state)
+                if known is None or new_cost < known:
+                    best_per_state[state] = new_cost
+                    current = results.get(nxt)
+                    if current is None or new_cost < current:
+                        results[nxt] = new_cost
+                    heapq.heappush(heap, (new_cost, nxt, new_parity))
+        return results
diff --git a/src/pf1e_simulator/los.py b/src/pf1e_simulator/los.py
new file mode 100644
index 0000000..fbbc4e2
--- /dev/null
+++ b/src/pf1e_simulator/los.py
@@ -0,0 +1,129 @@
+"""Corner-rule line of effect and cover.
+
+Squares are unit cells on an integer lattice: cell (row, col) spans
+x in [col, col+1], y in [row, row+1]. A line is blocked when its RELATIVE
+INTERIOR touches a blocking square's closure (interior, border, or vertex);
+lines merely starting or ending on a blocking vertex do not count. All math
+is exact integer arithmetic: parameter t is kept as (num, den) pairs, no
+floats anywhere.
+"""
+
+from __future__ import annotations
+
+from typing import TYPE_CHECKING
+
+if TYPE_CHECKING:
+    from collections.abc import Callable
+
+    from pf1e_simulator.grid import Grid
+    from pf1e_simulator.map import Pos, TerrainType
+
+Pt = tuple[int, int]  # lattice point (x, y) = (col, row)
+_Rat = tuple[int, int]  # exact rational as (numerator, denominator), den > 0
+
+
+def _corners(pos: Pos) -> tuple[Pt, Pt, Pt, Pt]:
+    row, col = pos
+    return ((col, row), (col + 1, row), (col, row + 1), (col + 1, row + 1))
+
+
+def _norm(num: int, den: int) -> _Rat:
+    return (num, den) if den > 0 else (-num, -den)
+
+
+def _le(a: _Rat, b: _Rat) -> bool:
+    return a[0] * b[1] <= b[0] * a[1]
+
+
+def _lt(a: _Rat, b: _Rat) -> bool:
+    return a[0] * b[1] < b[0] * a[1]
+
+
+def _slab(p1: Pt, delta: Pt, lo: int, hi: int, axis: int) -> tuple[_Rat, _Rat] | None:
+    """Closed t-interval where p1 + t*delta lies within [lo, hi] on `axis`."""
+    start = p1[axis]
+    step = delta[axis]
+    if step == 0:
+        return ((0, 1), (1, 1)) if lo <= start <= hi else None
+    t1 = _norm(lo - start, step)
+    t2 = _norm(hi - start, step)
+    return (t1, t2) if _le(t1, t2) else (t2, t1)
+
+
+def _segment_hits_cell(p1: Pt, p2: Pt, row: int, col: int) -> bool:
+    """True iff the relative interior of p1->p2 touches the closed cell."""
+    delta = (p2[0] - p1[0], p2[1] - p1[1])
+    if delta == (0, 0):
+        return False
+    interval_x = _slab(p1, delta, col, col + 1, 0)
+    if interval_x is None:
+        return False
+    interval_y = _slab(p1, delta, row, row + 1, 1)
+    if interval_y is None:
+        return False
+    t_low = interval_x[0] if _le(interval_y[0], interval_x[0]) else interval_y[0]
+    t_high = interval_x[1] if _le(interval_x[1], interval_y[1]) else interval_y[1]
+    return _le(t_low, t_high) and _lt((0, 1), t_high) and _lt(t_low, (1, 1))
+
+
+class _LineGrid:
+    """Bound view for corner-line checks between two squares of one grid."""
+
+    def __init__(
+        self,
+        grid: Grid,
+        a: Pos,
+        b: Pos,
+        blocking: Callable[[TerrainType], bool],
+    ) -> None:
+        self._grid = grid
+        self._exclude = frozenset({a, b})
+        self._blocking = blocking
+
+    def blocked(self, p1: Pt, p2: Pt) -> bool:
+        min_x = min(p1[0], p2[0])
+        max_x = max(p1[0], p2[0])
+        min_y = min(p1[1], p2[1])
+        max_y = max(p1[1], p2[1])
+        for col in range(min_x - 1, max_x + 1):
+            for row in range(min_y - 1, max_y + 1):
+                pos = (row, col)
+                if pos in self._exclude or not self._grid.in_bounds(pos):
+                    continue
+                if self._blocking(self._grid.terrain_at(pos)) and _segment_hits_cell(
+                    p1, p2, row, col
+                ):
+                    return True
+        return False
+
+
+def _grants_cover(terrain: TerrainType) -> bool:
+    return terrain.cover or terrain.blocks_los
+
+
+def has_line_of_effect(grid: Grid, a: Pos, b: Pos) -> bool:
+    """LoE iff some corner of a reaches some corner of b unblocked."""
+    if a == b:
+        return True
+    lines = _LineGrid(grid, a, b, lambda terrain: terrain.blocks_los)
+    return any(
+        not lines.blocked(corner_a, corner_b)
+        for corner_a in _corners(a)
+        for corner_b in _corners(b)
+    )
+
+
+def has_cover(grid: Grid, attacker: Pos, target: Pos, *, ranged: bool) -> bool:
+    """Cover iff every attacker corner has at least one blocked target corner.
+
+    The attacker picks their single best corner (PF1e ranged cover rule);
+    Phase 0 documented deviation: melee reuses the same corner rule.
+    """
+    if attacker == target:
+        return False
+    _ = ranged
+    lines = _LineGrid(grid, attacker, target, _grants_cover)
+    return not any(
+        all(not lines.blocked(corner_a, corner_t) for corner_t in _corners(target))
+        for corner_a in _corners(attacker)
+    )
diff --git a/src/pf1e_simulator/map.py b/src/pf1e_simulator/map.py
new file mode 100644
index 0000000..d4676c0
--- /dev/null
+++ b/src/pf1e_simulator/map.py
@@ -0,0 +1,183 @@
+"""Map YAML loading and validation.
+
+Maps are multi-layer ASCII: a mandatory `terrain` layer plus optional
+`elevation` and `zones` layers of identical dimensions, named `markers`, and
+a `deployment` section binding sides to zone letters. One char = one square,
+row 0 is the top line of the ASCII block.
+"""
+
+from __future__ import annotations
+
+from typing import TYPE_CHECKING
+
+import yaml
+from pydantic import BaseModel, ConfigDict, Field, ValidationError
+
+if TYPE_CHECKING:
+    from pathlib import Path
+
+Pos = tuple[int, int]  # (row, col), 0-indexed
+
+_ELEVATION_CHARS = frozenset("0123456789.")  # digit = height, '.' = ground level
+
+
+class MapValidationError(Exception):
+    """Raised when a map YAML file fails validation; carries path and reason."""
+
+    def __init__(self, path: Path, reason: str) -> None:
+        super().__init__(f"{path}: {reason}")
+        self.path = path
+        self.reason = reason
+
+
+class TerrainType(BaseModel):
+    """What one square means for movement and sight lines."""
+
+    model_config = ConfigDict(frozen=True, extra="forbid")
+
+    type: str
+    move_cost: int | None = Field(default=None, ge=1)
+    cover: bool = False
+    blocks_los: bool = False
+
+
+class MapSpec(BaseModel):
+    """A fully validated map, ready for Grid construction."""
+
+    model_config = ConfigDict(frozen=True, extra="forbid")
+
+    name: str
+    square_size_ft: int = Field(default=5, gt=0)
+    terrain: tuple[str, ...]
+    legend: dict[str, TerrainType]
+    elevation: tuple[str, ...] | None = None
+    zones: tuple[str, ...] | None = None
+    markers: dict[str, Pos] = Field(default_factory=dict)
+    deployment: dict[str, str] = Field(default_factory=dict)
+
+
+class _MapFile(BaseModel):
+    """Raw YAML shape; cross-layer checks live in load_map."""
+
+    model_config = ConfigDict(extra="forbid")
+
+    name: str
+    square_size_ft: int = Field(default=5, gt=0)
+    terrain: str
+    legend: dict[str, TerrainType]
+    elevation: str | None = None
+    zones: str | None = None
+    markers: dict[str, Pos] = Field(default_factory=dict)
+    deployment: dict[str, str] = Field(default_factory=dict)
+
+
+def _split_rows(block: str, what: str, path: Path) -> tuple[str, ...]:
+    rows = tuple(line.rstrip() for line in block.splitlines() if line.strip())
+    if not rows:
+        msg = f"{what} layer is empty"
+        raise MapValidationError(path, msg)
+    if len({len(row) for row in rows}) != 1:
+        msg = f"{what} layer has ragged rows"
+        raise MapValidationError(path, msg)
+    return rows
+
+
+def _check_layer_dimensions(
+    rows: tuple[str, ...], what: str, height: int, width: int, path: Path
+) -> None:
+    if len(rows) != height or len(rows[0]) != width:
+        msg = f"{what} layer dimensions do not match terrain"
+        raise MapValidationError(path, msg)
+
+
+def _optional_layer(block: str | None, what: str, path: Path) -> tuple[str, ...] | None:
+    """Split an optional ASCII layer, or None when the layer is absent."""
+    return _split_rows(block, what, path) if block else None
+
+
+def _check_elevation(elevation: tuple[str, ...], height: int, width: int, path: Path) -> None:
+    _check_layer_dimensions(elevation, "elevation", height, width, path)
+    bad = sorted({ch for row in elevation for ch in row if ch not in _ELEVATION_CHARS})
+    if bad:
+        msg = f"elevation chars must be digits or '.': {bad}"
+        raise MapValidationError(path, msg)
+
+
+def _check_markers(markers: dict[str, Pos], height: int, width: int, path: Path) -> None:
+    for marker_name, (row, col) in markers.items():
+        if not (0 <= row < height and 0 <= col < width):
+            msg = f"marker {marker_name!r} out of bounds: {(row, col)}"
+            raise MapValidationError(path, msg)
+
+
+def _check_deployment(
+    deployment: dict[str, str], zones: tuple[str, ...] | None, path: Path
+) -> None:
+    if not deployment:
+        return
+    if zones is None:
+        msg = "deployment declared but no zones layer"
+        raise MapValidationError(path, msg)
+    available = {ch for row in zones for ch in row if ch != "."}
+    missing = sorted({letter for letter in deployment.values() if letter not in available})
+    if missing:
+        msg = f"deployment zones absent from zones layer: {missing}"
+        raise MapValidationError(path, msg)
+
+
+def load_map(path: Path) -> MapSpec:
+    try:
+        text = path.read_text(encoding="utf-8")
+    except OSError as exc:
+        msg = f"cannot read file: {exc}"
+        raise MapValidationError(path, msg) from exc
+    try:
+        data: object = yaml.safe_load(text)
+    except yaml.YAMLError as exc:
+        msg = f"invalid YAML: {exc}"
+        raise MapValidationError(path, msg) from exc
+    try:
+        raw = _MapFile.model_validate(data)
+    except ValidationError as exc:
+        raise MapValidationError(path, str(exc)) from exc
+
+    terrain = _split_rows(raw.terrain, "terrain", path)
+    height, width = len(terrain), len(terrain[0])
+    unknown = sorted({ch for row in terrain for ch in row if ch not in raw.legend})
+    if unknown:
+        msg = f"terrain chars missing from legend: {unknown}"
+        raise MapValidationError(path, msg)
+
+    elevation = _optional_layer(raw.elevation, "elevation", path)
+    if elevation is not None:
+        _check_elevation(elevation, height, width, path)
+
+    zones = _optional_layer(raw.zones, "zones", path)
+    if zones is not None:
+        _check_layer_dimensions(zones, "zones", height, width, path)
+
+    _check_markers(raw.markers, height, width, path)
+    _check_deployment(raw.deployment, zones, path)
+
+    return MapSpec(
+        name=raw.name,
+        square_size_ft=raw.square_size_ft,
+        terrain=terrain,
+        legend=raw.legend,
+        elevation=elevation,
+        zones=zones,
+        markers=raw.markers,
+        deployment=raw.deployment,
+    )
+
+
+def zone_cells(spec: MapSpec, letter: str) -> list[Pos]:
+    """Cells of a deployment zone, sorted by (row, col); empty if no zones layer."""
+    if spec.zones is None:
+        return []
+    return [
+        (row, col)
+        for row, line in enumerate(spec.zones)
+        for col, ch in enumerate(line)
+        if ch == letter
+    ]
diff --git a/tests/test_grid.py b/tests/test_grid.py
new file mode 100644
index 0000000..642ffd9
--- /dev/null
+++ b/tests/test_grid.py
@@ -0,0 +1,114 @@
+"""Tests for the square grid: distance, step costs, corner rule, Dijkstra."""
+
+from __future__ import annotations
+
+from pf1e_simulator.grid import Grid
+from pf1e_simulator.map import MapSpec, TerrainType
+
+
+def make_grid(rows: list[str]) -> Grid:
+    legend = {
+        "#": TerrainType(type="wall", move_cost=None, blocks_los=True),
+        ".": TerrainType(type="floor", move_cost=1),
+        "T": TerrainType(type="rubble", move_cost=2),
+        "C": TerrainType(type="pillar", move_cost=None, cover=True),
+    }
+    spec = MapSpec(name="test", terrain=tuple(rows), legend=legend)
+    return Grid.from_spec(spec)
+
+
+def test_distance_diagonal_table() -> None:
+    grid = make_grid([".....", ".....", ".....", ".....", "....."])
+    assert grid.distance((0, 0), (1, 1)) == 1
+    assert grid.distance((0, 0), (2, 2)) == 3
+    assert grid.distance((0, 0), (3, 2)) == 4
+    assert grid.distance((0, 0), (0, 3)) == 3
+    assert grid.distance((2, 1), (0, 0)) == 2
+
+
+def test_step_cost_orthogonal_uses_entered_square() -> None:
+    grid = make_grid([".T."])
+    assert grid.step_cost((0, 0), (0, 1), 0) == 2
+    assert grid.step_cost((0, 1), (0, 2), 0) == 1
+
+
+def test_step_cost_diagonal_alternates() -> None:
+    grid = make_grid(["...", "...", "..."])
+    assert grid.step_cost((0, 0), (1, 1), 0) == 1
+    assert grid.step_cost((1, 1), (2, 2), 1) == 2
+    assert grid.step_cost((1, 1), (2, 2), 2) == 1
+
+
+def test_step_cost_diagonal_into_difficult_is_flat_three() -> None:
+    grid = make_grid(["..", ".T"])
+    assert grid.step_cost((0, 0), (1, 1), 0) == 3
+    assert grid.step_cost((0, 0), (1, 1), 1) == 3
+
+
+def test_diagonal_forbidden_past_wall_corner() -> None:
+    grid = make_grid([
+        ".#.",
+        ".#.",
+        "...",
+    ])
+    assert grid.diagonal_allowed((2, 1), (1, 0)) is False
+    assert grid.diagonal_allowed((2, 1), (1, 2)) is False
+
+
+def test_diagonal_allowed_with_both_flanks_clear() -> None:
+    grid = make_grid([
+        ".#.",
+        "...",
+        "...",
+    ])
+    assert grid.diagonal_allowed((2, 1), (1, 0)) is True
+
+
+def test_reachable_open_floor_costs() -> None:
+    grid = make_grid([".....", ".....", ".....", ".....", "....."])
+    reached = grid.reachable((0, 0), 10, frozenset())
+    assert reached[(0, 0)] == 0
+    assert reached[(1, 1)] == 1
+    assert reached[(2, 2)] == 3
+    assert reached[(3, 3)] == 4
+    assert reached[(4, 4)] == 6
+    assert reached[(0, 4)] == 4
+
+
+def test_reachable_respects_budget() -> None:
+    grid = make_grid([".....", ".....", ".....", ".....", "....."])
+    reached = grid.reachable((0, 0), 2, frozenset())
+    assert (1, 1) in reached
+    assert (2, 2) not in reached
+
+
+def test_reachable_diagonal_into_rubble_costs_three() -> None:
+    grid = make_grid(["...", ".T.", "..."])
+    reached = grid.reachable((0, 0), 6, frozenset())
+    assert reached[(1, 1)] == 3
+
+
+def test_reachable_through_rubble_cheaper_than_around() -> None:
+    grid = make_grid(["...", ".T.", "..."])
+    reached = grid.reachable((1, 0), 6, frozenset())
+    assert reached[(1, 2)] == 3
+
+
+def test_reachable_creatures_block_entry_not_diagonals() -> None:
+    grid = make_grid(["...", "...", "..."])
+    reached = grid.reachable((1, 1), 1, frozenset({(1, 0)}))
+    assert (1, 0) not in reached
+    assert reached[(0, 2)] == 1
+
+
+def test_reachable_paths_around_creatures() -> None:
+    grid = make_grid(["...", ".#.", "..."])
+    reached = grid.reachable((1, 0), 6, frozenset())
+    assert (1, 1) not in reached
+    assert reached[(1, 2)] == 4
+
+
+def test_reachable_cannot_enter_wall_squares() -> None:
+    grid = make_grid(["..", ".C"])
+    reached = grid.reachable((0, 0), 9, frozenset())
+    assert (1, 1) not in reached
diff --git a/tests/test_los.py b/tests/test_los.py
new file mode 100644
index 0000000..3b61095
--- /dev/null
+++ b/tests/test_los.py
@@ -0,0 +1,90 @@
+"""Tests for corner-rule line of effect and cover (los.py).
+
+Geometry convention: a line is blocked when its relative interior touches a
+blocking square (interior, border, or vertex); lines that merely start or end
+on a blocking square's vertex do not count as passing through it.
+"""
+
+from __future__ import annotations
+
+from pf1e_simulator.grid import Grid
+from pf1e_simulator.los import has_cover, has_line_of_effect
+from pf1e_simulator.map import MapSpec, TerrainType
+
+
+def make_grid(rows: list[str]) -> Grid:
+    legend = {
+        "#": TerrainType(type="wall", move_cost=None, blocks_los=True),
+        ".": TerrainType(type="floor", move_cost=1),
+        "T": TerrainType(type="rubble", move_cost=2),
+        "C": TerrainType(type="pillar", move_cost=None, cover=True),
+    }
+    spec = MapSpec(name="test", terrain=tuple(rows), legend=legend)
+    return Grid.from_spec(spec)
+
+
+def test_los_open_field() -> None:
+    grid = make_grid(["...", "...", "..."])
+    assert has_line_of_effect(grid, (0, 0), (2, 2)) is True
+
+
+def test_los_same_square() -> None:
+    grid = make_grid(["."])
+    assert has_line_of_effect(grid, (0, 0), (0, 0)) is True
+
+
+def test_los_blocked_by_wall_between() -> None:
+    grid = make_grid([".#.", "..."])
+    assert has_line_of_effect(grid, (0, 0), (0, 2)) is False
+
+
+def test_los_through_doorway_gap() -> None:
+    grid = make_grid(["##.##", "##.##"])
+    assert has_line_of_effect(grid, (0, 2), (1, 2)) is True
+
+
+def test_los_pinched_vertex_stays_open() -> None:
+    grid = make_grid([".#.", "#.."])
+    assert has_line_of_effect(grid, (0, 0), (1, 1)) is True
+
+
+def test_los_blocked_past_wall_line() -> None:
+    grid = make_grid(["...#.", "....."])
+    assert has_line_of_effect(grid, (0, 0), (0, 4)) is False
+
+
+def test_cover_open_field() -> None:
+    grid = make_grid([".....", "....."])
+    assert has_cover(grid, (0, 0), (0, 4), ranged=True) is False
+
+
+def test_cover_pillar_between() -> None:
+    grid = make_grid([".....", "..C..", "....."])
+    assert has_cover(grid, (1, 0), (1, 4), ranged=True) is True
+
+
+def test_no_cover_when_pillar_off_axis() -> None:
+    grid = make_grid(["..C..", "....."])
+    assert has_cover(grid, (1, 0), (1, 4), ranged=True) is False
+
+
+def test_no_cover_adjacent_with_wall_behind_target() -> None:
+    grid = make_grid(["..#"])
+    assert has_cover(grid, (0, 0), (0, 1), ranged=True) is False
+
+
+def test_cover_diagonal_past_wall_vertex() -> None:
+    grid = make_grid([".#.", "#.."])
+    assert has_cover(grid, (0, 0), (1, 1), ranged=True) is True
+
+
+def test_cover_melee_uses_same_corner_rule_in_phase0() -> None:
+    grid = make_grid([".#.", "#.."])
+    assert has_cover(grid, (0, 0), (1, 1), ranged=False) is True
+    open_grid = make_grid(["...", "..."])
+    assert has_cover(open_grid, (0, 0), (1, 1), ranged=False) is False
+
+
+def test_cover_same_square_is_false() -> None:
+    grid = make_grid(["."])
+    assert has_cover(grid, (0, 0), (0, 0), ranged=True) is False
diff --git a/tests/test_map.py b/tests/test_map.py
new file mode 100644
index 0000000..63a110e
--- /dev/null
+++ b/tests/test_map.py
@@ -0,0 +1,93 @@
+"""Tests for the map YAML schema (map.py)."""
+
+from __future__ import annotations
+
+from pathlib import Path
+
+import pytest
+
+from pf1e_simulator.map import MapValidationError, load_map, zone_cells
+
+MAPS_DIR = Path(__file__).resolve().parents[1] / "data" / "maps"
+
+
+def _write(tmp_path: Path, body: str) -> Path:
+    target = tmp_path / "map.yaml"
+    target.write_text(body, encoding="utf-8")
+    return target
+
+
+def _minimal_map() -> str:
+    return """
+name: "Test"
+terrain: |
+  ###
+  #.#
+  ###
+legend:
+  "#": { type: wall, blocks_los: true }
+  ".": { type: floor, move_cost: 1 }
+"""
+
+
+def test_sample_arena_loads() -> None:
+    spec = load_map(MAPS_DIR / "sample_arena.yaml")
+    assert spec.square_size_ft == 5
+    assert len(spec.terrain) == 8
+    assert all(len(row) == 20 for row in spec.terrain)
+    assert spec.legend["#"].move_cost is None
+    assert spec.legend["#"].blocks_los is True
+    assert spec.legend["C"].cover is True
+    assert spec.legend["T"].move_cost == 2
+    assert spec.deployment == {"players": "A", "monsters": "B"}
+    assert spec.markers["autel"] == (1, 16)
+
+
+def test_sample_arena_zones_and_elevation() -> None:
+    spec = load_map(MAPS_DIR / "sample_arena.yaml")
+    assert spec.elevation is not None
+    assert spec.elevation[1][2] == "1"
+    cells_a = zone_cells(spec, "A")
+    assert cells_a == [
+        (1, 1), (1, 2), (1, 3), (1, 4),
+        (2, 1), (2, 2), (2, 3), (2, 4),
+    ]
+    assert len(zone_cells(spec, "B")) == 8
+
+
+def test_minimal_map_without_optional_layers(tmp_path: Path) -> None:
+    spec = load_map(_write(tmp_path, _minimal_map()))
+    assert spec.elevation is None
+    assert spec.zones is None
+    assert spec.markers == {}
+    assert zone_cells(spec, "A") == []
+
+
+def test_rejects_ragged_rows(tmp_path: Path) -> None:
+    body = _minimal_map().replace("  #.#\n", "  #.##\n")
+    with pytest.raises(MapValidationError):
+        load_map(_write(tmp_path, body))
+
+
+def test_rejects_char_missing_from_legend(tmp_path: Path) -> None:
+    body = _minimal_map().replace("  #.#\n", "  #x#\n")
+    with pytest.raises(MapValidationError):
+        load_map(_write(tmp_path, body))
+
+
+def test_rejects_elevation_dimension_mismatch(tmp_path: Path) -> None:
+    body = _minimal_map() + "elevation: |\n  000\n  00\n"
+    with pytest.raises(MapValidationError):
+        load_map(_write(tmp_path, body))
+
+
+def test_rejects_marker_out_of_bounds(tmp_path: Path) -> None:
+    body = _minimal_map() + "markers:\n  autel: [9, 1]\n"
+    with pytest.raises(MapValidationError):
+        load_map(_write(tmp_path, body))
+
+
+def test_rejects_deployment_without_zone(tmp_path: Path) -> None:
+    body = _minimal_map() + "zones: |\n  ...\n  .A.\n  ...\ndeployment:\n  players: B\n"
+    with pytest.raises(MapValidationError):
+        load_map(_write(tmp_path, body))