Rathaus-Marienturm – Late-Roman Castrum and Carolingian Remains

The name Aquae Granni, the Roman precursor of modern-day Aachen, points to Roman Aachen’s greatest asset: its hot springs! The mineral waters were quickly believed to have healing powers. Granni refers to Grannus, a Gallic deity associated with health and recovery. The earliest thermal baths date to around the turn of the millennium. From the outset, the Romans envisioned Aachen on a grand scale: even at its founding, the town had the air of a true city – 30 hectares in size, with wooden houses, thermal baths, and even a stone bathhouse with tiled roofs.

A City Growing Around Its Baths

In the early second century, Aquae Granni underwent a complete urban renewal. Emperor Trajan may have intended to elevate the vicus to a regional capital – a theory supported by stamped bricks and an inscription bearing his name. The original baths made way for a monumental square of more than 6,000 square metres, likely containing a forum and temple. Across the city, monumental new bath complexes were built.

Aquae Granni became one of the Roman Empire’s leading wellness centres and the only major spa in Germania Inferior.

Rathaus-Marienturm – Castrum and Carolingian Legacy

As the city grew around its baths and public spaces, defensive and administrative structures became increasingly important. The Rathaus-Marienturm offers a rare glimpse into the late-Roman castrum and its medieval evolution. In a side room of the town hall, parts of the castrum’s defensive wall are visible in the foundation, including reused worked stones (spolia), such as fragments of a Roman milestone. Additionally, the masonry of the Carolingian apse of the aula regia and the remains of the castrum’s interior buildings can be seen.

Although the site is currently not accessible to the public, a digital presentation is planned to allow visitors to explore these remains on site. This location demonstrates the continuity of settlement in Aachen, linking the city’s late-Roman fortifications to its medieval administrative and ceremonial structures.

Discover Roman Aachen on the VIA VIA Route, 2027

The Rathaus-Marienturm is a key stop on the VIA VIA Roman City Walk route of Aachen, connecting the city’s museum exhibit in Centre Charlemagne with its archaeological remains. Along the trail, you can explore other highlights—from Roman walls and floors in the Elisengarten to the Münsterthermen inside Aachen Cathedral. Informative panels, multimedia displays, and encounters with Roman “characters” provide context, illustrating Roman administration, urban development, bathing culture, and the late-Roman castrum. The Rathaus-Marienturm underscores how Aachen’s Roman heritage continued to shape the city through the medieval period, linking public, civic, and defensive architecture across centuries.

Are you ready to take a walk?

“‘The city smells of rotten eggs,’ Julia grimaces. My daughter is right. The thermal baths of Aquae Granni may be healing, but their sulphurous stench is unbearable. It doesn’t seem to bother the bathers, though. Our cart moves forward at a crawl through a crowd of shuffling people and wagons piled high with wood. ‘Most of it is for the baths,’ says a dark-skinned man walking beside us. ‘Every day, the thermae and the underfloor heating of the rich devour another piece of the forest. The baths heal the body, but not nature,’ he sighs, before vanishing again into the throng. Quintus Iulius has come up with the idea of bottling the spring water and selling it at the markets of Germania Inferior. Personally, I’m just looking forward to sliding into the warm water for a few hours — and talking with people from all corners of the Empire. Perhaps I’ll overhear some news about the situation in Rome. They seem to change emperors every few months this year. Murder and decapitation — not even the most curative water can wash that away.” – Ammulva Iucunda