Simple treehouse plan
To fit the door, screw the hinges to the door first. A strip of 2x1 is used on the hinge edge of the door to provide a solid fixing point for the hinges. Get someone to hold the door in place against the treehouse frame (or use pieces of wood) and screw the other side of each hinge to the corner of the frame through the plywood of the end wall. Attach a sliding bolt and catch, preferably with security bolts, ie bolts which can only be removed from the inside.