I was trained by Luke Ellison so I rely on foamboard myself. But I also built on his original hallway/corridor building. If you want the walls to sit up easily, you can use L-brackets bought from Lowes or Home Depot. I have a bunch of extras since taking apart my sets. Mind you, you use those, you might as well use a wood base so the setting won't warp or buckle.

I admit, the one thing that foamboard does NOT do is show texture. A friend at work who plays around with doing these sort of things too (she's an anime freak), she uses spackle to cover the walls to give them texture then paints over them. It supposedly keeps the foamboard from warping. *shrug* But I like your use of cardstock!

I could've sworn I did a how-to of how I do things, but it seems to have disappeared from my blog... Anyway... Very good.