Urkulu (1,419 m) is a mountain in the Basque Country straddling the border between France and Spain. The main feature of the mountain is the remains of a Roman tower topping the summit, erected in the 1st century BC. to commemorate the recent conquest of Aquitaine.
With the summit providing excellent views over the northern and southern slopes alike, it was used as a watchtower in medieval times.
The tower has a truncated-conical shape, measures 19.5 meters in diameter at the base and is 3.6 meters high. Its original height should have been 4.5 meters. The thickness of the walls is 2.6 meters and its interior is filled with the remains of the original stonework.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.