This playhouse was made entirely out of pallets I found around town. I used 2" and 3" wood screws. Total cost was about $6.00 and 1 weekend. It was great using pallets because it was more like free form assembling than actually building.
My son built this playhouse fort from treated wood pallets. It took him and the boys only about 3 hours to build. They may add a roof later as he is able to get more wood pallets. Third picture down on the right shows an inside view with a ladder to climb to the upper level. Bottom right picture shows a small balcony on the upper level.