Yep. I use the castle pieces fairly often, but they do require MOO, and they do have to be applied rather late. Not so late that you have no idea what the place is going to look like, but late enough that the walls are already there, if you're going to want walls.
It's tricky. And then you have to worry about routing, because if the wall pieces overlap something that you want to make functional, there can be problems. So you have to make extra sure to test everything, to make sure you won't get routing errors.