>>9887920>>9887965Indeed!
I think the next new interior backgrounds were in "Prince Adam No More" (someone correct me if I'm wrong). When Beast Man captures King Randor, he takes him to an entrance behind Blood Falls. It leads into this area, made up of a score of these concave wall panels punctuated with stalagmites.
They trip a laser alarm halfway, alerting Mer-Man at a guard post.