Here are a couple of castles my son and I have created over the years. They are completely handmade from cardboard or foam core. They look complicated but are easy to make. In addition to cardboard and foam core they require just a few tools, including an Xacto knife, straight edge, hot glue/glue gun, and a bit of paint. Foam core is easy to shape. Scoring it allows it to bend so that you can create shapes like the towers. We even included a working portcullis in the first one! Painting each little stone onto the walls took us a long time, but it was well worth it.