The Marschiertor can be visited
City Wall Aachen-Marschiertor

The Marschiertor in Aachen, a work of medieval defensive architecture, ranks among the most powerful preserved city gates in Western Europe. As the southern component of the outer city fortifications, it once shaped the cityscape and served as a strategic access point.
History+
Construction of the Marschiertor began around 1257 and was completed shortly after 1300. It was the main southern gate of Aachen's second city wall ring, known as the Gothic Wall, which was built from the 14th to the 15th century. Originally, it had an outer gate, which was demolished in the 17th century to make way for modern fortifications. The gate served various purposes, including as an armory, storage space, homeless shelter, and youth hostel.
Source: Wikipedia · Licence: CC BY-SA 4.0 · Image: http://de.wikipedia.org/wiki/Datei:Marschiertor_Aachen.jpg
This description draws on the facts documented in the linked Wikipedia article; Wikipedia content is licensed under CC BY-SA 4.0. Wording and editing: Mobile History.
Location: 50.7682, 6.0869