Children love to crawl into things. So why not build something that is easy to build and to lift, when it needs to be transported, and that has a magical appeal to toddlers? So, the Knights Castle from cardboard was born...

Supplies

Cardboard boxes from big things
Scissors
Cutter knife
Package tape with dispenser
Ruler
Paint and brush
Accessories like washi tape, stickers, old keys, ...

Step 1: Choose and Get Cardboard Boxes

The bigger the better was what I thought about the boxes. Of course their shape and size should fit our flat, too. And if their width is as wide that the kids can easily crawl through it but not change their direction that easy when crawling it could be more fun and challenging for them entering the castle.

My boxes were from a circular saw and other stuff I don't remember. Ask in a local electronics store or at another vendor of big things :-)

Step 2: Cut Doors, Windows, a Drawbridge, Other Openings Into It

The kids want to enter the castle. The regular way was over the drawbridge, right? So our cardboard castle got one. A high box was put standing in front of a longer box and fixed to each other with lots of tape. The opening for the drawbridge was cut into the high box with the sharp cutter knife. Take care, it's really sharp! No way doing these steps together with the kids... the cardboard can be very sharp as well!

The castle's front door, that's the drawbridge at the same time, is only at the bottom side not cut. I fixed two thin ropes (yellow) on the left and right upper corners of the door with knots and some tape and made the rope pass through some drinking straws that had been fixes to the board with tape. On top of the long box there's a small hole to put the ropes through into the inner room of the castle. Inside you can put a wooden bead onto the ropes ends to allow the kids to close the bridge when pulling the bead.

Step 3: Expand Your Castle With More Rooms, Doors, Towers ...

... entrance, gate, bridges, ...

Of course, a real castle or a castle has a lot of rooms. Our place in the flat is a bit limited, and that's why I built the three-room variant. But there were two more big and a few smaller towers. The trick is that children - depending on their size - can stand in the towers and look out. Optionally, they throw cushions, cuddly toys, or other objects in or out, or cover the towers with a corrugated cardboard box that came from a grapefruit or orange box and made me remember storm clouds. Works well with the game kids play in the castle!

I cut the towers with the cutter knife above so that they look like the battlements of a real castle, and they protrude a little over the tower. This was easy to do by attaching a piece of cardboard with the cut out battlements to the existing tower.

In the beginning the kids could enter the long box and another one (from the circular saw) that was put rectangular to the long one. Later I added a tower and another long box behind the saw box. Inside there were now two cardboard doors that allowed crawling in a circle and leaving the castle with the face forward. Also, two (or three small) kids now could play inside at the same time. Much better now!

Step 4: Decoration, Ornament, Jewelry, Painting, Outfitting...

Now it's your part decorating the castle in the way your creativity wants to. For me this part wasn't the main interest... so you still can see cardboard, tape and that may occur a little ugly. My daughter tried to cover some parts with washi tape. Sure you could do this step even more accurate than I could! For example you could cover only the outer line of the castle's shape with washi tape.

I built some roofs over the towers with a sheet of stiff cardboard. The columns were made from rolls from toilet or napkin papers used in the kitchen.

In the front door cardboard box we've added a "door lock" - drilling a hole in the cardboard and masking the hole with some gold foil was enough for the girl. Putting a pretty, old key inside and being able to turning it made the game more interesting for her.

Step 5: Some Last Thoughts...

Finally, with a few hours of "work", a real castle was built up out of a few large boxes. At least for the kids, it was a real castle... "was", because we have given it away in the meantime.
A word on durability - over three years, the children played in the castle, they were between two and five years old. But even older children were in it and had a lot of fun. For the assembly I had to go in a lot of times - and somehow I always came out again :-)

When doing it again, I would do everything the same way, but with one or two other persons together because of more fun when working together. And these other persons would add some finishing steps and make the castle look more pretty :-)

Hope you have some fun or even inspiration to build your own (or your kid's) cardboard castle. It's really fun!!