We collected toilet paper cardboard tubes (I might have pestered some freinds to get more faster. They are used to me rifling through their recyling now) and a couple of snack boxes (I think they were Annies cheddar bunny snacks or something, but any cracker type box/cereal box will do)
Here's a scribble showing how we cut the turrets out of the loo roll tubes and another scribble showing how to squeez four large walls and four small walls out of a snack box.The kids painted the castle pieces first and then while they were drying we painted the base with brown and green and a nice blue moat to surround the castle.
Then everyone ate lunch and ran about like they were deranged while the base dried.
We glue gunned the castle pieces together and then the kids played with a load of toy animals and cars in the castle (I think we might have to have a project where we make knights and peasants and a king and queen to actually go in the castle, because as entertaining as it obviously was, driving cars around the moat and storming the castle with a giant polar bear is neither historically or geographically accurate.)