diff --git a/src/card.rs b/src/card.rs
index 879b8e6..bd796a0 100644
--- a/src/card.rs
+++ b/src/card.rs
@@ -191,6 +191,60 @@ mod test {
     assert!(extra_doors.is_none());
   }
 
+  #[test]
+  fn check_merge_doors() {
+    let bottom = Card {
+      cells: NW_TRIANGLE,
+      s: true,  // Test that this is not overwritten by an empty tile
+      se: true, // Test that this is subsumed by another door
+      e: true,  // Test that this is popped off into the second card
+      ..Default::default()
+    };
+
+    let top = Card {
+      cells: [
+        Cell::Empty,
+        Cell::Empty,
+        Cell::Empty,
+        Cell::Empty,
+        Cell::Empty,
+        Cell::Filled,
+        Cell::Empty,
+        Cell::Empty,
+        Cell::Filled,
+      ],
+      se: true, // Test that this is subsumed by another door
+      n: true,  // Test that is is dropped because no floor
+      ..Default::default()
+    };
+
+    let expected = Card {
+      cells: [
+        Cell::Empty,
+        Cell::Empty,
+        Cell::NW,
+        Cell::Empty,
+        Cell::NW,
+        Cell::Filled,
+        Cell::NW,
+        Cell::Filled,
+        Cell::Filled,
+      ],
+      s: true,
+      se: true,
+      ..Default::default()
+    };
+
+    let expected_doors = Card {
+      e: true,
+      ..Default::default()
+    };
+
+    let (stacked, extra_doors) = bottom.merge(top);
+    assert_eq!(stacked, expected);
+    assert_eq!(extra_doors, Some(expected_doors));
+  }
+
   /// Merging two triangular [`Cell`]s should prefer the top-most one, *unless*
   /// they are opposites. See test [`merge_opposite_triangles()`].
   #[test]
@@ -275,10 +329,10 @@ mod test {
   }
 
   #[test]
-  fn transpose_horizontal(){
+  fn transpose_horizontal() {
     todo!();
   }
-  
+
   #[test]
   fn rotate_clockwise() {
     todo!();