If you want to build a seriously strong door, employ drawbore mortise and tenon joinery. It's a technique that uses a wooden dowel to greatly strengthen a mortise and tenon joint, transforming it from a joint that relies only on glue adhesion into a joint that has a permanent and mechanical interlock. It works by drilling a hole through the tenon that is slightly offset from a hole in the mortise, and when you drive a dowel through the two holes, the dowel seeks to align the two holes, thus tightly pulling the joint together. I used two dowels for each mortise and tenon joint, and there's something especially pleasing about ramming a square piece of wood through a round hole to make your own dowels.
