Cardboard is not the greatest material for making wargames buildings — formcore is far better. But for making ruins, it actually works quite well. I built enough ruins to cover a 5×5 table for a Stalingrad game from cardboard. Terragenesis has an article on making ruins from cardboard that look remarkably like the ones I made.
Chechen Conflict Rules
Here’s a one-page set of free wargames rules for the Chechen Conflict, with figures based on on stands.
Modeling Walls and Barracades
Here’s a nice video on modeling walls and barracades for wargames. The techniques are particularly post-apocalyptic.
Flat Out War Universal Rules
Flat Out War, from Oversoul Games is touted as a set of universal rules for 25mm miniatures. It’s for “experienced gamers,” since it does not explain or illustrate a lot of common terminology.