The Castle of Archidona was built by the Arabs in the 9th century and rebuilt by Alhamar, the first Nasrid king, in the 13th century. In 1462 Archidona was conquered by the Conde de Osuna, beginning to expand outside the walls to occupy the area it currently has. After the Christian conquest, the new population center and its most prominent urban representation in the Plaza Ochavada, the 18th century curious octagonal shape is established; in her red brick and white lime are combined.References:
The Petersberg Citadel is one of the largest extant early-modern citadels in Europe and covers the whole north-western part of the Erfurt city centre. It was built after 1665 on Petersberg hill and was in military use until 1963. It dates from a time when Erfurt was ruled by the Electors of Mainz and is a unique example of the European style of fortress construction. Beneath the citadel is an underground maze of passageways that can be visited on guided tours organised by Erfurt Tourist Office.
The citadel was originally built on the site of a medieval Benedictine Monastery and the earliest parts of the complex date from the 12th century. Erfurt has also been ruled by Sweden, Prussia, Napoleon, the German Empire, the Nazis, and post-World War II Soviet occupying forces, and it was part of the German Democratic Republic (East Germany). All of these regimes used Petersberg Citadel and had an influence on its development. The baroque fortress was in military use until 1963. Since German reunification in 1990, the citadel has undergone significant restoration and it is now open to the public as a historic site.