diff --git a/src/pf1e_simulator/map.py b/src/pf1e_simulator/map.py
index 444c888..32e9f56 100644
--- a/src/pf1e_simulator/map.py
+++ b/src/pf1e_simulator/map.py
@@ -183,6 +183,13 @@ def load_map(path: Path) -> MapSpec:
 
     terrain = _split_rows(raw.terrain, "terrain", path)
     height, width = len(terrain), len(terrain[0])
+    multi_char = sorted(key for key in raw.legend if len(key) != 1)
+    if multi_char:
+        msg = (
+            "legend keys must be exactly one character (one char = one "
+            f"square), got: {multi_char}"
+        )
+        raise MapValidationError(path, msg)
     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}"
diff --git a/tests/test_map.py b/tests/test_map.py
index 51a3cbc..8144379 100644
--- a/tests/test_map.py
+++ b/tests/test_map.py
@@ -93,6 +93,26 @@ def test_rejects_char_missing_from_legend(tmp_path: Path) -> None:
         load_map(_write(tmp_path, body))
 
 
+def test_rejects_multi_character_legend_key(tmp_path: Path) -> None:
+    """One char = one square: a legend key like "ZZ" can never match a cell
+    and is dead data -- reject it at load."""
+    body = _minimal_map().replace(
+        '  ".": { type: floor, move_cost: 1 }\n',
+        '  ".": { type: floor, move_cost: 1 }\n  "ZZ": { type: floor, move_cost: 1 }\n',
+    )
+    with pytest.raises(MapValidationError, match="legend keys must be exactly one character"):
+        load_map(_write(tmp_path, body))
+
+
+def test_rejects_empty_legend_key(tmp_path: Path) -> None:
+    body = _minimal_map().replace(
+        '  ".": { type: floor, move_cost: 1 }\n',
+        '  "": { type: floor, move_cost: 1 }\n',
+    )
+    with pytest.raises(MapValidationError, match="legend keys must be exactly one character"):
+        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):