What is it called & how do you decorate it?

What is it called when you have living room walls that don't connect to the ceiling? It's hard to explain, but my living room has 3 walls with a Spanish style opening as the entrance. One of the walls doesn't completely connect because it turns into sort of a vault ceiling & is an opening to the kitchen on the top of the wall? I hope someone knows what I'm talking about, but it's really been bugging me because I don't know how to make use of this wall. I know I could put plants up there or some kind of knick knacks, but I don't want it to look tacky so any suggestions would be helpful!