The good thing about the walls are that they show how big the city was 1700 years ago, when emperor Aurelian built them. Their function was to keep the city safe from barbarian raids - a function they did not accomplish to the fullest. They are pretty thick and very tall - about 9 meters, and encompass the city from all sides, one end at Villa Borghese, other at Laterano, and you can (restored) parts of them while going sightseeing. Best kept gates are Porta San Giovanni at Lateran end and Porta Pinciana at Colle Pincio end. You can not miss them.