Take several cereal boxes, cut one side open and then tape back together inside out (so the brown cardboard inside shows on the outside.). You can cut castle details, glue on windows or architectural details. You can add cones or toilet paper rolls and make it as fancy as you like.
I think the kids favorite part was making the flags. I brought up a picture of all the various countries' flags and they drew them with markers on small rectangle pieces of paper. Then they taped them to toothpicks and I helped tape them onto the castle. They loved doing it and also learned what a lot of different countries' flags look like. So simple and you just use supplies around the house.
