Scoring, then snapping, hard plastic strucutures has always given me better results than any kind of saw ever did.
This may be a limitiation of my own skill and/or patience, but sawing styrene always ended up with a crooked, ugly cut. Even perfectly aligned cuts tend to ruin one side or the other of the piece, and it sure would be great to save the house part AND that nice tower from your garage sale find...
The razor saw may work as well as scoring for you, but scoring it has been more wriggle proof for me.
For cutting a straight line on a flat surface, you need:
*a scribe/etcher/small awl (Amazon, hardware stores sell them under several names)
*a steel ruler or square
*a clamp (optional)
Clamp the ruler where you want the cut, or hold it in place, and just gently run the scribe along the edge.
If your first pass is where you need it, just repeatly run the scribe over the groove, putting just a bit more pressure on with each pass.
After you are about 1/3 to 1/2 of the way through, the plastic should snap nicely along the seam you just made.
If you are cutting at a corner, say, where a flat wall meets a tower wall, you don't even need a ruler, just use the inside corner as you guide.
Sometimes you may still need to use a saw to cut underlying supports from walls. Using a scribe on the outer surface can keep the visible face clean even if things are messier behind the wall.
HTH. Sorry the tip was late.