The central keep serves as a multi-story climbing tower with staggered internal floor platforms allowing cats to leap from the ground level up to the roof lookout deck. The exterior can be finished with hand-drawn stone brick textures using cat-safe non-toxic markers. The color scheme pairs natural kraft cardboard brown with black ink brick detailing.

+-------------------------------------------------------------+
|              3-TIER MEDIEVAL CARDBOARD FORTRESS             |
|                                                             |
|   +----+  +-------------------------------------+  +----+   |
|   | T1 |  | BATTLEMENTS & ROOF LOOKOUT DECK     |  | T2 |   |
|   |    |  +-------------------------------------+  |    |   |
|   +----+  | SECOND STORY SLEEPING CHAMBER       |  +----+   |
|   |    |  +-------------------------------------+  |    |   |
|   | T3 |  | GROUND LEVEL LOUNGE & DRAWBRIDGE    |  | T4 |   |
|   +----+  +=====================================+  +----+   |
|                 \  Fold-Down Drawbridge Ramp /              |
+-------------------------------------------------------------+

Step-by-Step DIY Assembly

  1. Prepare the Base Keep: Join two large heavy-duty wardrobe or appliance boxes vertically, cutting a 8-inch circular portal in the middle floor panel for internal leaping.
  2. Attach Corner Turrets: Glue four tall, narrow shipping boxes vertically to the four outer corners of the central keep using non-toxic hot glue along interior seams.
  3. Cut Crenellated Battlements: Cut 2×2 inch square notches along the top edges of the roof panels to form classic medieval castle battlements.
  4. Install Drawbridge Entry: Cut an arched door on the front wall, leaving the bottom edge attached to form a fold-down drawbridge ramp anchored with natural jute twine.
  5. Insert Scratching Pad Floor: Glue a corrugated cat scratching insert onto the ground floor entry level.