Roman Walls was constructed in the 6th century BC. It was to prevent a repeat of the sack of Rome during Battle of the Allia by the Gauls of Brennus. Roman Walls were still standing in the end of the Republic and the early Empire. By then Rome had begun to expand outside the wall. Rome continued to grow and thrive. And as the military grew and got stronger the Roman Walls became unwarranted.
Roman Walls were built from monstrous blocks of tufa. Tufa consists of volcanic rock made from ash and rock particles spewed out during an eruption.
Most of the Roman Walls did not endure the time since it was built. Parts of the Roman walls can be seen around the city. Perhaps your first view being on the train arriving from the airport near the entrance of the Termini station. Another notable section is at the end of Via Veneto as you approach the Villa Borghese. You can see small sections as you pass Piazza Magnanapoli just above Trajan's Forum. My favorite segment was when I took a stroll up the Aventine to visit the church of Santa Sabina and the other basilicas on the hill.
Near the Roman walls which you see next to Via Veneto, you can then visit Via Veneto directly. If you cross the street you'll walk straight inside Villa Borghese Park and if you walk further you'll see Galleria Borghese and then the Bioparco (zoo).
If you stay on Via Veneto and walk down the road, you'll hit the Church Santa Maria della Concezione on your left hand side and then Piazza Barberini, which has the Triton fountain standing in the middle of it. If you turn the corner to your left hand side, you'll see Barberini Palace.
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