Quick answer: In an apartment, build a compact two-tower cardboard castle about 24 × 16 inches using two or three plain boxes, cut doorways 6–7 inches wide, join boxes with paper tape or PVA glue instead of plastic tape and staples, and place it near a window or the sofa. Kids can design, measure and decorate while an adult does the cutting.
A cardboard castle is a perfect apartment weekend project. It costs almost nothing, uses the delivery boxes that pile up by the door, makes no loud noise and gives an indoor cat a new place to hide, climb and nap. Kids get a real design task with a happy customer at the end. The only apartment-specific challenge is space: a sprawling three-box fortress that works in a house will block a small living room, so the plan below is built around a compact footprint.
Indoor cats benefit from new places to explore and hide; Ohio State's Indoor Pet Initiative lists hiding spots and climbing options among an indoor cat's basic needs. A castle you can collapse and store under the bed when guests visit fits that need without permanently taking up floor space.
The compact apartment plan
The design is two towers side by side: a lower 'hall' box laid on its side as the ground floor, and a taller 'keep' box standing upright behind it with a window at cat-head height. Together they take about 24 × 16 inches of floor — roughly the size of a large cushion. If you have more space, a third small box on top of the hall makes a lookout.
| Part | Box size (approx.) | Job |
|---|---|---|
| Hall (ground floor) | 16 × 12 × 12 in | Entrance tunnel and hiding spot |
| Keep (tower) | 16 × 16 × 20 in | Upper window and nap shelf |
| Lookout (optional) | 12 × 12 × 8 in | Top platform for confident cats |
Medium moving boxes are often about 18 × 18 × 16 inches and work well as the keep. Shoe boxes are too flimsy to bear weight but are fine as decoration such as turrets.
Collecting the right boxes
Start collecting a week ahead. You want plain corrugated cardboard: brown or white, without a glossy plastic-coated print, without wax coating (common on produce boxes) and in good shape — not damp, not crushed at the corners. Double-wall boxes, which have two layers of fluting, are much stronger for anything a cat will stand on. Ask neighbors or your building's recycling area; apartment buildings produce plenty of boxes.
Before the build day, an adult strips each box: peel off plastic packing tape, remove shipping labels, pull out any staples with pliers, and discard any boxes that smell of cleaning products or food. This is the single most important safety step, and children can help by checking for missed staples with a fridge magnet once the adult has finished.
Jobs for kids and adults
| Job | Who | Notes |
|---|---|---|
| Sketching the castle | Kids | Draw it on paper first; adults approve door sizes |
| Tracing doors and windows | Kids | Use a dinner plate for doors, a cup for peepholes |
| Cutting openings | Adult | Box cutter with a fresh blade, on a cutting mat |
| Folding flaps into ramps or shelves | Kids 6+ | Crease along a ruler |
| Joining boxes | Adult with kid help | PVA glue and paper tape |
| Decorating | Kids | Washable markers or crayons on outside walls |
| Final safety check | Adult and kids together | Magnet sweep for staples, push test |
In a small flat, keep the cutting on a single board or mat on the kitchen table, and put each cut-out scrap into a bag immediately. Cardboard scraps on the floor are a trip hazard for kids and a tempting chew toy for the cat.
Doors, windows and ramps
Cats prefer openings they can pass through without squeezing, but small enough to feel enclosed. A round door 6–7 inches wide suits most adult cats; go 8 inches for large breeds. Place the main door low, about 1 inch above the floor, and add a second exit on another side so the cat never feels trapped — cats are more comfortable in hiding places with an escape route. Windows of 3–4 inches at cat-head height on the keep give a lookout.
To connect the hall and the keep, cut matching 6-inch openings where the boxes touch, then glue the boxes together around the opening. A flap folded inside the keep at about 10 inches high, supported by a smaller box underneath, becomes a nap shelf. Smooth any rough cut edges by rubbing them with the side of a pencil or a scrap of cardboard.
Quiet, apartment-friendly joining
Skip the staple gun and the hot glue gun. Staples work loose and end up in paws or mouths, and hot glue strings can be picked off and swallowed. In a flat, both also make noise or fumes. Instead, use PVA school glue or wood glue on the contact surfaces, clamp with clothes pegs or heavy books for an hour, and reinforce outside corners with brown paper tape — the kind that is activated with water. Paper tape is quiet, cat-safe and looks like part of the cardboard.
- PVA glue lines on every contact surface; let it dry for at least an hour, ideally overnight.
- Water-activated paper tape on outside seams.
- Slot-and-tab joints: cut a slot in one box and a tab on the other for a glue-free hold.
- Avoid plastic packing tape, duct tape and staples.
Where it fits in a small home
Place the castle where your cat already likes to watch the room: beside the sofa, next to a window with a view, or at the foot of the bed. Avoid hallways, where it blocks traffic, and keep it at least 3 feet from heaters, radiators or candles. If floor space is tight, put the castle against a wall so only the front and one side are open. More ideas for cat spots in small homes are in small-space cat furniture and cat furniture that doesn't look like cat furniture.
Most castles will become a scratching target too. That is fine — cardboard is a texture many cats love. If your cat seems to prefer horizontal scratching, you will see it clawing the castle's floor; vertical vs horizontal scratchers explains what that tells you.
Introducing the cat
Let the cat discover the castle on its own terms. Kids will want to put the cat inside — explain that this usually makes cats avoid the castle. Instead, drop a few treats near the entrance, put a familiar blanket on the nap shelf, and drag a wand toy past the doorways. International Cat Care on play recommends short, frequent play sessions that let the cat stalk and pounce, and a castle with two doors is a perfect stage for this. Many cats explore within an hour; shy cats may wait until the house is quiet at night.
A Saturday-to-Sunday timeline
Saturday morning: sketch the castle at breakfast and agree on the door sizes. Saturday late morning, in polite apartment hours: the adult cuts openings while kids trace the next ones, then glue the boxes and weigh the joints down with books. Saturday afternoon: leave everything to dry while you go out — glue sets faster without little fingers testing it. Sunday morning: decorate the outside walls, fold in the nap shelf and do the magnet sweep. Sunday afternoon: place the castle, hide treats and step back. Splitting the job this way keeps each session under an hour, which suits young attention spans, and means the cat meets a finished, dry, glue-free castle. For a neat way to make cat gear sit nicely in a grown-up room afterwards, see adult-looking cat living rooms.
Collapsing and storing
The beauty of cardboard in a flat is that it disappears. If you used slot-and-tab joints, the castle comes apart in minutes and the pieces slide under a bed. Glued castles can be folded at the box seams if you plan for it. Keep the best pieces and recycle anything chewed, damp or soiled. A castle usually lasts a few weeks to a couple of months before it needs replacing — and that becomes the next weekend project.
If your cat loves cardboard, a sturdier permanent scratching option nearby keeps them off the sofa between castles. See the complete sisal scratcher guide and how to get your cat to use a scratcher.
Apartment cardboard castle FAQ
How big should a cardboard cat castle be in an apartment?
A two-box design of about 24 × 16 inches works in most small flats. Add a small lookout box on top if you have the floor space.
What size doorway does a cat need?
A round opening of 6–7 inches suits most adult cats; use about 8 inches for large breeds. Add at least two exits.
Is packing tape safe on a cat castle?
Avoid it. Plastic tape can be chewed and swallowed. Use PVA glue, water-activated paper tape or slot-and-tab joints instead.
How long does a cardboard castle last?
A few weeks to a couple of months, depending on scratching. Replace sections that are damp, soiled or chewed.
