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.
Step 2: Get it ready
Draw the crenellations and door.
Step 3: Cutting the crenels.
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.
#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.