Deus meus! this city is overwhelming! Follow my posts as I attempt (and not for the first time) to get some sort of handle on it for myself!
The Servian Wall was constructed around the city of Rome in the early 4th century BC. The wall was up to 10 metres (32.8 ft) in height in places, 3.6 metres (12 ft) wide at its base, 11 km (7 mi) long, and is believed to have had 16 main gates, though many of these are mentioned only from writings, with no other known remains.
The map below show the Servian Wall and its gates in Blue.
The Aurelian Walls (red) were constructed in the 3rd century AD.
The Servian wall was built from large blocks of tuff (a volcanic rock made from ash and rock fragments ejected during an eruption) quarried from the Grotta Oscura quarry near Rome’s early rival Veii. In addition to the blocks, some sections of the structure incorporated a deep fossa, or ditch in front of it, as a means to effectively heighten the wall during attack from invaders.
Along part of its topographically weaker northern perimeter was an agger, a defensive ramp of earth heaped up to the wall along the inside. This thickened the wall, and also gave defenders a base to stand while repelling any attack. The wall was also outfitted with defensive war engines, including catapults.
The wall was still maintained through the end of the Republic and in the early Empire. By this time, Rome had already begun to grow outside the original Servian Wall. The organization of Rome into regions under Augustus placed regions II, III, IV, VI, VIII, X, and XI within the Servian Wall, with the other sections outside of it.
The wall became unnecessary as Rome became well protected by the ever-expanding military strength of the Republic and of the later Empire. As the city continued to grow and prosper, it was essentially unwalled for the first three centuries of the Empire.
When German tribes made further incursions along the Roman frontier in the 3rd century AD, Emperor Aurelian had the larger Aurelian Walls built to protect Rome.