Daddy?

Yes?

Can I borrow that box? I need to do an Instructable?

Yes, #2 son has done an Instructable. He's only six, so be nice. (ahem) It's his first Instructable...

Step 1: What You Need.

You need a big box, a big pen, a big pair of scissors, and the last of Daddy's duct tape.

Step 2: Get It Ready

Stick the flaps together with the duct tap to make the box taller.

Draw the crenellations and door.

Step 3: Cutting the Crenels.

Crenellations are important. It's not a castle if it doesn't have crenellations.

Did you know that the crenels are the gaps, not the sticky-up bits?

#2 son found it easier to cut all the vertical cuts first, then bend bits over to cut them off.

Step 4: The Door.

Daddy...

#2 son decided I should use my Instructables knife to cut the door.

Holes got poked so he can pull the doors shut from inside.

Lumps of card duct-taped to the inside of the doors made slots to put a bar across the doors.

Step 5: That's It!

For now, the castle is finished, but #2 Son has plans - he's thinking about painting it to be more realistic (real castles are painted white inside), and is keeping his eye out for more card to make a drawbridge...