Furstenburg Castle was erected in the 13th century on behalf of the Bishop Conrad of Chur (1272-1282). In the 16th and 17th centuries A.D. it was however restructured according to the style of the time. The oldest part of the castle is the tower which displays walls of a three meters thick diameter.References:
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.