Museum of the Walls. This museum is housed in the twin towers of Porta San Sebastiano, the largest, most important, and best-preserved city gate in the walls built by the emperor Aurelianus to defend Rome from the barbarians. Begun in the 3rd century AD, the walls stretched for about 19 km (12 mi) around the city, encompassing the much-earlier Servian wall. A century later, after a new wave of invasions, the walls were restored and doubled in height. The museum has some interesting displays on how the walls were built and added to over the centuries. Restorations of a section of the wall that includes nine towers and a covered gallery are under way. Call ahead to find out the current status: this area is a highlight of a visit here, with some unexpected views of the gardens and greenery that still exist amid the walls. COST: EUR2.50.