If Pariah folk doesn't do anything for you it appears to have influences of Genghis Kahn's Palace so look at ancient Mongolian architecture for stuff. It will be similar, but different ways of looking at it. If you want a motte and bailey sieged by orcs I would say put tall towers with banners on them. Make them crude and jagged. Play around with destroyed architecture. If orcs sieged it then damage will show, but the orcs would have added their own supports for the damaged buildings. Twisted fantasy inspired beams and things like that. Remember, orcs and banners go hand in hand being a war mongering race.