Wyloch has some good tips here for modeling ruined corners. Corners are among the strongest parts of a building, so they will naturally be prominent parts of ruins.
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.
Osprey is supporting their popular Frostgrave rules with some paper model ruins for the game. They could of course also be used for a number of other medieval and fantasy games.