diff --git a/src/AcDream.Core/Physics/PhysicsEngine.cs b/src/AcDream.Core/Physics/PhysicsEngine.cs
index 802b470..36dacfc 100644
--- a/src/AcDream.Core/Physics/PhysicsEngine.cs
+++ b/src/AcDream.Core/Physics/PhysicsEngine.cs
@@ -187,7 +187,13 @@ public sealed class PhysicsEngine
 
             if (crossedPortal is not null)
             {
-                // Outdoor → Indoor: enter the OwnerCellId.
+                // Outdoor → Indoor: enter the OwnerCellId IF the target cell
+                // actually contains the candidate position. Without CellBSP,
+                // we verify by checking that SampleFloorZ returns non-null
+                // (position is within the cell's floor polygon bounds) AND the
+                // floor Z is close to the player's current Z (not a basement
+                // 30m below). This prevents the wall-bounce bug where portal
+                // planes on upper floors captured outdoor positions.
                 uint enterCellIndex = crossedPortal.Value.OwnerCellId & 0xFFFFu;
                 CellSurface? enterCell = null;
                 foreach (var c in physics.Cells)
@@ -195,8 +201,24 @@ public sealed class PhysicsEngine
                     if ((c.CellId & 0xFFFFu) == enterCellIndex) { enterCell = c; break; }
                 }
                 float? enterFloorZ = enterCell?.SampleFloorZ(candidatePos.X, candidatePos.Y);
-                targetZ = enterFloorZ ?? terrainZ;
-                targetCellId = enterCellIndex;
+
+                // Validate: floor must exist AND be within step height of current Z.
+                // This rejects transitions to basements, upper floors, and cells
+                // whose floor polygon doesn't actually cover this position.
+                bool validTransition = enterFloorZ is not null
+                    && MathF.Abs(enterFloorZ.Value - currentPos.Z) < stepUpHeight + 2f;
+
+                if (validTransition)
+                {
+                    targetZ = enterFloorZ!.Value;
+                    targetCellId = enterCellIndex;
+                }
+                else
+                {
+                    // Portal crossed but target cell doesn't contain us — stay outdoor.
+                    targetZ = terrainZ;
+                    targetCellId = physics.Terrain.ComputeOutdoorCellId(localCandX, localCandY);
+                }
             }
             else
             {