Sadly, I think I've only seen the Aurelian Walls (and don't even remember it properly, overwhelmed as I was with all the sights and sounds in the city)...
The world has seen many wars and conflicts throughout history, which has given us a multitude of ancient defensive structures, like fortified walls, to see and explore. Here listed are just some of the many historic walls that litter the earth. They are all impressive feats of architecture for their time of construction and boast the pride and strength of their ancient civilizations.
You can access the list here.