Roman Walls (Lugo)

Located on a site above the Minho river and named after the Celtic god Lugus, it is the only city in Europe to be surrounded by completely intact Roman walls, which reach a height of 10 to 15 metres along a 2'5 km circuit ringed with 71 towers. The walk along the top is continuous round the circuit. These 3rd century walls are protected by UNESCO as a World Heritage Site. The bridge over the Minho is essentially of Roman date, though many repairs over the centuries have effaced its Roman character.

More info at  http://en.wikipedia.org/wiki/Lugo