The museum of the Roman city walls may not be the most famous of Rome, but it is worth a visit. Situated in the ancient Porta Appia, now known as the Porta di San Sebastiano, it offers a lot of information about the history of the defenses of the eternal city.
You enter the museum through the western tower. The two main rooms are situated over the gate itself, and when you look through the windows, you can see what the soldiers must have seen when they operated the catapults. In these two rooms, texts and photos explain the history from the age of Romulus until 1870: Roma Quadrata, the Servian Wall, the Aurelian Wall, and the medieval expansions surrounding the Borgo and connecting it to Trastevere.
The rooms in the eastern tower offer a couple of models, while a small room upstairs above the entry contains stone decorations from medieval towers. From here, you can ascend to the crenellations and the roof of one of the towers. The terrace offers you a splendid view of the Aurelian wall, the Parco degli Scipioni and the Roman countryside, dominated by the Alban Mount.
Perhaps you will remember that some ten years ago, part of the southeastern sector of the Aurelian Wall collapsed after some heavy rains. You will see photos of the situation in the 1990s, after the collapse, and after the repairs. An inscription commemorating earlier repairs by pope Innocent X and his coat of arms are now on display.
In general, a nice museum, although perhaps more for people fascinated by army matters than for a more general audience. A visit can easily be combined with a trip to the Via Appia, which starts at this very gate.