diff --git a/src/card.rs b/src/card.rs
index e91f728..1340851 100644
--- a/src/card.rs
+++ b/src/card.rs
@@ -1,5 +1,7 @@
 //! TODO: module doc :v
 
+use std::ops::Deref;
+
 /// Value for the "sub tiles" inside a room tile
 #[derive(Clone, Copy, Debug, Default, PartialEq)]
 enum Cell {
@@ -50,11 +52,11 @@ enum RotationDir {
 #[derive(Clone, Copy, Debug, Default, PartialEq)]
 struct Card {
   cells: [Cell; 9],
-  e: bool,
-  ne: bool,
-  n: bool,
   nw: bool,
+  n: bool,
+  ne: bool,
   w: bool,
+  e: bool,
   sw: bool,
   s: bool,
   se: bool,
@@ -63,7 +65,53 @@ struct Card {
 impl Card {
   /// Produces a new card by stacking another on top of this one.
   pub fn merge(self, top: Self) -> (Self, Option<Self>) {
-    todo!();
+    let mut new_card = Self::default();
+    let mut doors: Option<Self> = None;
+
+    new_card.cells = std::iter::zip(&self.cells, &top.cells)
+      .map(|(lower, upper)| {
+        if (lower == &Cell::Filled || upper == &Cell::Filled)
+          || (lower == &Cell::SW && upper == &Cell::NE)
+          || (lower == &Cell::SE && upper == &Cell::NW)
+          || (lower == &Cell::NE && upper == &Cell::SW)
+          || (lower == &Cell::NW && upper == &Cell::SE)
+        {
+          Cell::Filled
+        } else if upper != &Cell::Empty {
+          *upper
+        } else {
+          *lower
+        }
+      })
+      .collect::<Vec<Cell>>()
+      .try_into()
+      .unwrap();
+
+    // Check for doors
+    macro_rules! check_door {
+      ( $dir:ident, $index:literal ) => {
+        if self.$dir {
+          if top.cells[$index] == Cell::Empty || top.$dir {
+            new_card.$dir = true;
+          } else {
+            doors.get_or_insert_default().$dir = true;
+          }
+        } else if top.$dir && new_card.cells[$index] != Cell::Empty {
+          new_card.$dir = true;
+        }
+      };
+    }
+
+    check_door!(nw, 0);
+    check_door!(n, 1);
+    check_door!(ne, 2);
+    check_door!(w, 3);
+    check_door!(e, 5);
+    check_door!(sw, 6);
+    check_door!(s, 7);
+    check_door!(se, 8);
+
+    (new_card, doors)
   }
 
   /// Cuts this Card on the given line. Returns two cards
@@ -124,9 +172,15 @@ mod test {
     // For now, triangular cells from the top-most Card survive the merge.
     let expected = Card {
       cells: [
-        Cell::NW, Cell::Filled, Cell::NE,
-        Cell::Filled, Cell::Filled, Cell::Filled,
-        Cell::SW, Cell::Filled, Cell::Filled,
+        Cell::NW,
+        Cell::Filled,
+        Cell::NE,
+        Cell::Filled,
+        Cell::Filled,
+        Cell::Filled,
+        Cell::SW,
+        Cell::Filled,
+        Cell::Filled,
       ],
       ..Default::default()
     };