Rome's walls define it in myriad ways. They defined the city, and how it could protect itself - or not - from the 4th century BC to 1870 and even to the present. No second trip to Rome should miss an up-close-and-personal experience with the walls. The best way is the city's Wall Museum, located inside the enormous, twin-towered, San Sebastiano gate. You can poke around, go up into the towers, walk along inside the walls... a small, but nifty thrill. These walls, of course, are the newer ones - the Aurelian walls, built in 271-275 AD (Wikopedia has some good historical facts.) There are only a few pieces of the 4th century BC Servian walls left in Rome (see around Termini - the central train station).
The museum's location means you also get to walk in and around the old Roman road, now via di Porta San Sebastiano - the beginning of the famed Appian Way (via Appia Antica). You can also pair this Top 40 #38 with #39, graffiti - which is just outside the gate.
Photos here are of the gate (above - inside; below, outside the gate).
The museum entrance is on the inside, right (as you look out) of the gate. Hours 9-2 Tuesday - Sunday, ticket office closes at 1:30. Tickets generally Euro 3. There's a lot of history and some old photos (but who needs old photos, when you've got "old" right in front of you??) on the city's website for the museum and gate (in English).
There's also a well-groomed, newly refurbished family park just before the gate. Parco degli Scipioni.
An alternative to the Wall Museum for seeing Rome's walls is the museum of Porta San Paolo near the Pyramid (on Itinerary 4 in Rome the Second Time).