Lugo

Lugo needs 1 day: walking the 2.2 km Roman wall that fully encircles the old town, the only one in the world entirely intact and standing, in a single day. May-September brings the mildest weather. It's inexpensive by Galician standards. Built between the 3rd and 4th centuries, it's been a UNESCO World Heritage Site since 2000, and it's the only city on the planet completely surrounded by an intact Roman wall, fully walkable along the top via a pedestrian path.