This is the other project that I've (re)started. I had the basics of it started a while back - the base. I noticed I had that, plus a lot of foam bits to use up from older projects. So I started with a simple wall on the base. That looked decent (better when painted) so I decided to build a removable tower that I could put inside the walls. 

It's made with your standard pink insulation foam (1" thick I believe) - I bought a sheet of this years ago and I'm still using it up! The tower is built with a cardboard box as its core to keep everything more or less straight.
 
There's no ground level entrance - the plan is to build a wooden stair going up to a 2nd/3rd level entrance - apparently this was fairly common, in an emergency everyone could go into the tower and burn/destroy the stair behind them....

Still have to put another window in it, and figure out how I'm going to handle the roof - you can sort of see the supports I put in for roof floor, just haven't decided what to make it out of yet.