Steve and I cut a window out of the wall today so I can put in a ~12" deep medicine cabinet. In the process, we got a good look at the structure of the wall which is steel lath, about an inch of concrete, and then some plaster. Overall, a very messy experience. The wall ate my oscillating tool, but we got through with an angle grinder. The first pic shows the tile, and the wall before the cut (that's the existing hole from the old cabinet) the second is the piece we removed.