Use PVC pipes. Great fort idea! Could stake it in the ground for a few things and put it away in the winter!
DIY collapsible pup tents! Wood strips, pvc pipe, twin flat sheet! Tons of fun!
An awesome fort building kit for kids by Fort Magic! This kit makes all-kinds of BIG, sturdy forts and other things (submarines, cars, castles) using tubes and connectors. (+ enter the #giveaway for a fort kit thru 3/18)
Rabbit something? pvc pipe fort diagram by It's Great To Be Home, via Flickr
Pvc pipe play pin
PVC Playhouse with the actual tutorial instead of link to only a pic
PVC pipe loft - let's see someone do this in copper.
How to build an indoor playhouse or fort. So cool!
This is a video and has the best cutting dimensions for interchangeable pieces.