Hi, I don't know if it'll help you, but I made a tutorial of making the plastic door from the mines of moria starterkit in cardboard. It's in my WIP, I'll add the link here. If you think it's to small for a fortress, you can make it larger with the same tutorial, just have more centimeters, as long as the proportions stay the same, you should be fine
If you'd like to add a door, I already have an idea for that, just tell me and I'll show you
Good luck!
http://www.one-ring.co.uk/viewtopic.php?f=50&t=19749&start=80