diff --git a/src/pf1e_simulator/map.py b/src/pf1e_simulator/map.py
index 95f6fe0..444c888 100644
--- a/src/pf1e_simulator/map.py
+++ b/src/pf1e_simulator/map.py
@@ -131,6 +131,13 @@ def _check_deployment(
     deployment: dict[str, str], zones: tuple[str, ...] | None, path: Path
 ) -> None:
     if not deployment:
+        if zones is not None:
+            msg = (
+                "zones layer declared but no deployment section (the README "
+                "requires deployment when zones exist: each zone letter needs "
+                "a side)"
+            )
+            raise MapValidationError(path, msg)
         return
     if zones is None:
         msg = "deployment declared but no zones layer"
diff --git a/tests/test_map.py b/tests/test_map.py
index d89793d..51a3cbc 100644
--- a/tests/test_map.py
+++ b/tests/test_map.py
@@ -111,6 +111,12 @@ def test_rejects_deployment_without_zone(tmp_path: Path) -> None:
         load_map(_write(tmp_path, body))
 
 
+def test_rejects_zones_without_deployment(tmp_path: Path) -> None:
+    body = _minimal_map() + "zones: |\n  ...\n  .A.\n  ...\n"
+    with pytest.raises(MapValidationError, match="no deployment section"):
+        load_map(_write(tmp_path, body))
+
+
 def test_rejects_shared_deployment_zone_letter(tmp_path: Path) -> None:
     """Two sides bound to the same zone letter would deploy on top of each
     other (build_states resets its placed set per side): a data error."""