The Ent Horde power reads:

Would I be correct that stacked Ents would not be able to destroy walls, since, while walls behave like blocking terrain they aren't actually "squares of" blocking terrain?

That is correct. Which is kinda sad in my opinion.

Ent: "We can destroy the thick stuff, but that thin, wimpy stuff is beyond us!"

Thanks, that's what I thought. This will make Swamp Thing's and Groot's new tree army not quite as effective if they get stuck indoors, but I think they'll manage.

The walls are obviously sprayed with Round-Up.