With very few exceptions, every single building in the game, from small stone huts to larger brick houses and even windmills, can be severely damaged or destroyed. This makes attack and defence of different areas of the map an evolving conflict. Snipers pinning you down from the second floor of that windmill? Remove the second floor from the equation. Enemies bunkering down in that mansion? Blow some extra holes in the walls to breach from. Mortar crews taking cover in a compound? Flatten the walls of the compound, leaving nowhere to hide.
In terms of big changes to the map, there are a couple with bridges that can be destroyed, preventing vehicle access to some areas of the map. One in particular (can't remember the name but it's the one with a central fort) has a capture point in the centre of the map that can be completely cut off to vehicles by destroying a stone bridge.
There are also the behemoths of course; both the airship and armoured train will leave persistent wreckage where they are destroyed, which can aid or aggravate your assault. On maps where the armoured train travels through the centre very close to the control points, the burning wreckage can be useful for defence and a good pilot will make sure the train is in an advantageous position when it is destroyed.