That's it; dump the plastic castle and keep the box. In a big cardboard box you can serve tea, plot war, hide from the enemy. You can cut windows and doors or slits for arrows. You can't carve your will on plastic or if you do your Mum and Dad will be angry and worry you'll grow up to be a vandal.
Another thing: I was hanging up the washing the other morning and spread a King-sized sheet across the two laundry lines to dry. And there it was- the secret tent- the shadowy sides moving mysteriously in the breeze. So go green, employ the latest sophisticated solar technology,(hanging out your clothes in the Florida sunshine) and give your child an extra, organic hide out.
Not everyone has the space for a tree and a dog or a laundry line but everyone with a child must find room for the big cardboard box.