The toy was originally a GI Joe playset, but when I first saw it I recognized its potential for a 28mm spacecraft repair facility.
Having had a couple gifted to friends for their projects, I recognized it immediately, but had to re-read the text four times before I found you already knew.
For once, checked sufficiently before sounding the idiot. At least, a total idiot.
Let me say, I've never seen it better utilized. Very handsome and evocative.
Also, for the first time, I see how useful, for making retro shuttles, one could find toy barn silos...

Two pieces? The smaller doesn't ring a bell from what I recall of the fort. Are they attached to the flat? I assumed that was just a mat, to be replaced by table top, where you could spread more terrain as James so correctly suggests.
Doug