Not wasted! Even if none of us share in it, perhaps somebody might find a valuable technique or something to use. I for one, like how you used the bevel to represent the arrow slits. I agree that the coral does make for some interesting lichens.
I wouldn't worry about the walls vs. the rubble. Maybe the walls had some sort of plaster on them that was broken away when the wall crumbled. I might have thinned the walls a little where there is rubble next to them, but that isn't such a big deal.
Overall, I like it.