Cats like boxes. Our one, simply loves them. I think she also loves paperboard (carton). In this Instructable, me and my brother built a big paperboard castle for the Animal Innovations contest...and for our very cute and beloved cat.

Step 1: Getting Ready.

Gather your Supplies.

Lots and lots of paperboard (you will also need one big box as your base building)
Duct tape (or other tape but I think duct tape works the best)
Scissors
A staple device
A pencil
A knife

Step 2: Making the Base of the Castle.

Take your big box and flip it open, tape the sides together to make a taller box (that is if you want a taller box).

Step 3: Making Your Castle Towers.

Cut out a piece of paperboard that is as tall as you want your towers. Then trace lines where you will fold the paperboard into a tower. The width of the towers will be defined by how much space you leave between the lines. Make sure you have 4 spaces (which will become the sides of the tower) plus an extra piece, which will help to strengthen the tower, and then it will be easier to tape the sides together.

Step 4: Fold Your Paperboard Into a Square Shaped Tower.

Fold the paperboard and trim it to your liking. IMPORTANT: The last picture is how you want the base of your tower to look like. If you don't cut and fold it like that, you might have some problems sticking it unto your castle base.

Step 5: Now Cut Some Windows.

Here you can use your own creativity and imagination.

Step 6: Cut Some Battlements Into the Tower Top.

Step 7: Finally Duct Tape the Whole Tower Together.

Step 8: And Now... Repeat Those Steps Another 3 Times.

You might want to make only two, or even 5, or 6, or 7, or...

Step 9: Make Your Castle Base's Roof.

Cut out a piece of paperboard which is slightly bigger than your castle base. Fold the excess to make the roof fit nicely on the castle top.

Step 10: Then Duct Tape the Roof On.

Step 11: Adding the Castle Towers.

Simply put them on and duct tape them firmly. Duct tape is wonderful stuff...and its quite powerful.

Step 12: Add Some Battlements Between the Towers.

These really add a medieval effect to your castle.

Step 13: Add a Door.

Cut it out using a sharp knife. The size of the door depends on the size of your cat. If you have a JUMBO cat make a JUMBO door, if you have a tiny cat make a tiny door.

Step 14: Add Some Windows.

Again use your sharp knife to cut out some windows. The quantity of windows is your choice. Be creative.

Step 15: Cut a Flap That Can Open in the Top.

You can also add a handle as we did.

Step 16: Use the Staple Device to Add Strength.

Your paperboard might be to thick for this. But if not, it would help to add a few staple shots.

Step 17: Place a Smaller Box in the Castle.

The smaller box is like the throne.

It also makes your cat much cosier but it is not a necessity.

Step 18: Finishing Touches.

I added a few flags (4 to be exact). But here you can be as creative as possible. You can add decorations, interesting medieval artifacts, you can paint it, etc.