The Ponttor is one of the two remaining gates of the original city wall of Aachen. It was built in the 14th century and manned by soldiers and militia throughout the Free Imperial City of Aachen era. Today, the Ponttor is used by German Youth and Scout groups.
The Ponttor was constructed as a right-angled three-stock tower castle. In the main entrance, there is a Portcullis as well as a machicolation, through which things could be dropped on invading forces. A bridge passage with crenelations spanned a moat and was strengthened in the foregate with two reinforced towers (barbicans). The building material was Devonian sandstone, Pennsylvanian carbonite sandstone and quartzite, with the framing material made of light bluestone.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.