These polystyrene castings are purported to be compatible with 15-20mm miniatures, but I believe they would work for 25mm miniatures as well.
Proxie Models produces a whole line of products for the war-gamer in sizes from 15mm to 28mm. Proxie Models feature: vehicles, terrain pieces, and bases for your miniatures.
So far I have ordered two of their selections. Both structure ruins are cast in a hard polystyrene plastic and look like they will very easy to assemble.
Ruined brick building; package contains 2 spruces.
Ruined brick corner building; package contains 2 spruces, with second floor section.