If you've been to Gen Con, you probably know of Hirst Arts. By reputation, if not by name. They're the folks with the table in the back covered with bright blue/green silicone molds for making your own dungeons and castles and whatnot. After years of temptation, I finally bought some, and they really are a lot of fun. Think Legos, only cheaper and you're encouraged to break, cut, sand, and otherwise abuse the pieces. Bruce (the guy) has a bunch of info on his site: basic technique, pre-designed projects (the Descent gameboard looks great!), and the like.
After a bit of tinkering with some of the plans there, I decided to embark on my own design project. Something simple at first. How about modular fortress walls that I can piece together into arbitrary shapes? Broad, and tall. I wanted something that would handle horses riding on it. Or a line of archers to repel the invading hordes. Or both at once. Making such a thing truely to scale would be a bit silly - who wants a real 60' high wall, even at 1" to 5' scale? It's big and heavy, takes a long time to make, and would probably get knocked over during play. Not to mention you'd never see your players on the other side! So I decided to make my pieces one wall section (mold #200) tall. That's 3½", more than high enough to be imposing on a battlemat. So, without further ado, here are my instructions to build modular fortress walls.