Well, you could represent tough, but destructable walls, by giving them a high armour value, making them vulnerable to siege artillery, and giving them a couple of dozen hitpoints. And setting their speed to 0. Technically.

Of course, the AI won't like it one bit, but unless someone actually tries it out, we can't ever be sure. I think the Pike&Musket mod includes something similar in the for of "war vagons". So there is a precendent.