There are many ways to build a castle with children. Once I already made a small cardboard toy castle with my kids, but this time we wanted something more impressive. For our castle we collected cardboard boxes of different size, and toilet paper tubes. Using those building blocks, paint and some colored paper, we created this castle with floors, round towers, windows and a cone-shaped roof.
This project is very easy and straightforward. Reverse the boxes, stack them one onto another, put the toilet paper rolls at the corners, paint. Place towers also at the top and around the gate. I showed how to make a cone in my post about gigantic pencils, the cone-shaped roof can be made the same way. A flag from a toothpick and piece of paper is the final touch.
I think you may enjoy the Cardboard castle by Ann Wood. It's more of a piece of art than a kids craft project. Take a look.