I managed to destroy the Great Wall by bombarding the Arab town of Najran with trebuchets. This oddity was in a Monarch game with patch 1.13b. All Arab towns lost their walls right after the bombard message appeared. Apparently defenders don't have to be redlined to damage improvements, population and great wonders. This could be a big problem.

It is random whether you hit unit, pop, or buildings. The Problem here is that wonders can be destroyed at all. I believe they are aware of this.