You may have seen this in the background, but here's a quick documentation
I hacked up a wall section from GW Warhammer Fortress (I also have the full set -- slightly differnt color palette -- painted but got a second...)
(sorry for the crap lighting and photos in advance)
I made a 1/4" plywood base/insert
the slope of the ruined wall is such that most figures can "stand" at an angle...
the tower consists of 2 exterior walls + 2 exterior walls as interior walls with 1/4" plywood in between...green stuff on edges
plywood "base" floor glued to vertical walls then covered with planks for levels 2 & 3
the ground floor is a separate plywood square with a bit on an extension for the doorway...
so there it is....comments & questions welcome as always...