In 1828 Nicholas I of Russia mandated the building of the sea fortress of Patarei. Completed in 1840, it is located on area of 4 hectares (10 acres). Over the years it has had different functions. Since 1867 Patarei functioned as barracks and in 1920 it was moved as a prison. It housed inmates until 2004, and has been left virtually untouched since. Visitors can explore the hallways to see cells, work areas, exercise yards and the like. There are also several tours available, see website for details.
Reference: Official website, In Your Pocket
The city walls of Avila were built in the 11th century to protect the citizens from the Moors. They have been well maintained throughout the centuries and are now a major tourist attraction as well as a UNESCO World Heritage Site. Visitors can walk around about half of the length of the walls.
The layout of the city is an even quadrilateral with a perimeter of 2,516 m. Its walls, which consist in part of stones already used in earlier constructions, have an average thickness of 3 m. Access to the city is afforded by nine gates of different periods; twin 20 m high towers, linked by a semi-circular arch, flank the oldest ones, Puerta de San Vicente and Puerta del Alcázar.