The Church of Saint Catherine is a late Byzantine church in the northwestern corner of the Ano Poli, Thessaloniki. The church dates to the Palaiologan period, but its exact dating and original dedication are unknown. From its interior decoration, which survives in fragments and is dated to ca. 1315, it has been suggested that it was the katholikon of the Monastery of the Almighty.
It was converted to a mosque by Yakup Pasha in the reign of the Ottoman sultan Bayezid II (r. 1481–1512) and named after him Yakup Pasha Mosque. In 1988, it was included among the Paleochristian and Byzantine monuments of Thessaloniki on the list of World Heritage Sites by UNESCO.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.