diff --git a/squirrelbattle/mapgeneration/randomwalk.py b/squirrelbattle/mapgeneration/randomwalk.py
index 3ef2b2d..b913d1c 100644
--- a/squirrelbattle/mapgeneration/randomwalk.py
+++ b/squirrelbattle/mapgeneration/randomwalk.py
@@ -107,10 +107,11 @@ class Generator:
         # post-processing: add walls
         for x in range(width):
             for y in range(height):
-                c = sum([1 if grid[j][i] == Tile.FLOOR else 0 for j, i in result.large_neighbourhood(y, x)])
-                if c == 4 and self.params["no_lone_walls"]:
-                    result.tiles[y][x] = Tile.FLOOR
-                elif c > 0:
-                    result.tiles[y][x] = Tile.WALL
+                if grid[y][x] == Tile.EMPTY:
+                    c = sum([1 if grid[j][i] == Tile.FLOOR else 0 for j, i in result.large_neighbourhood(y, x)])
+                    if c == 4 and self.params["no_lone_walls"]:
+                        result.tiles[y][x] = Tile.FLOOR
+                    elif c > 0:
+                        result.tiles[y][x] = Tile.WALL
 
         return result