Cardboard seems like a pretty good building material for wargame terrain - it's dirt cheap (or free), fairly sturdy, and even comes in useful shapes (i.e. boxes -> buildings). But it also has a nasty tendency to warp when exposed to too much moisture, and the corrugation itself tends to make the edges a problem.

This is just a test - I took two pieces and glued them together to get a little more strength. I glued a couple bricks cut from thin card on to break up the plane of the wall a bit. Then, to cover the ends I took a piece of normal paper (from a notepad actually) and soaked it in watered down white glue. Once it was good and soaked, I applied it to cover each end of the wall. It worked pretty well to cover where the corrugation shows through. I think I'll try using masking tape for this in the future. I then mixed some light grey craft paint with some textured craft paint and coated the wall. Then, to get a little more texture, I mixed the light grey paint with some black, and sort of dabbed it on with a cheap foam brush. This made a dark-grey texturing effect, that helped break up the colors of the wall. The "bricks" are painted one with a thin brush. I thought of using a sharpie permanent black marker, but though it might not work well on the texture, and of course I couldn't find it either...

I think it turned out ok. I need to come up with a way to make the brick "courses" straighter and more even.

This walker was actually started by a friend of mine, who never completed it - so I've decided to finish it. He put on all the cool looking battle damage. I believe it's by Armorcast (it's resin) but I'm not totally sure. Still have a ways to go, but it's getting there.