The Alteburg Fleet Fort – Headquarters of the Roman Rhine Fleet in Cologne-Marienburg

The fleet fort is not a destination in the usual sense but a historical location with no visible remains. Its roughly 16-hectare river terrace now lies beneath the densely built-up Alteburg area in eastern Marienburg, about eight metres above the former Rhine floodplain and thus safe from flooding. Its approximate extent can only be traced through the surrounding streets. The camp's original Latin name has not survived.

A single structure still links the place directly to its past: the Alteburg mill (An der Alteburger Mühle 6), built at the end of the 18th century, stands to this day on the grounds of the former fort.

Naval units had been part of the Roman forces on the Rhine from around 12 BCE, though at first only as improvised detachments subordinate to the legions. After Rome's shift in Germanic policy under Tiberius in 16 CE, the ships mainly served river patrols and the transport of building materials. Around the middle of the 1st century the naval force – now first identifiable as the Classis Germanica – moved to the Cologne region.

Several phases can be distinguished at the site itself. As early as the first half of the 1st century – after the Battle of the Teutoburg Forest – legionary detachments were stationed here, probably of the Legio XX Valeria Victrix, building a camp of about twelve hectares. On this spot the smaller fleet fort of six to seven hectares was erected in late Claudian times, offering room for more than 1,000 men. It began as a pure timber-and-earth camp; between 90 and 100 at the latest its defences, and in the 2nd century its interior buildings, were rebuilt in stone. The camp lay just under three and a half kilometres south of the Colonia Claudia Ara Agrippinensium (CCAA).

The layout did not follow the usual "playing-card" scheme but adapted to the terrain on its western and southern sides, producing a polygonal shape. On the Rhine side the timber-phase defences were absent – the ground dropped steeply to the river, and the harbour installations were apparently part of the fort's protection. The later stone wall of opus caementicium with greywacke facing enclosed the camp on all sides, fronted by a pointed ditch 5.6 metres wide. The north and west gates have been identified.

Inside, the more recent excavations under Thomas Fischer distinguished up to eleven building phases between the years 9 and 270. With the fleet came the typical long, rectangular barracks with their contubernia. Remarkably, the enlisted men's barracks carried elaborate interior wall paintings otherwise known only from officers' quarters. The finds document a wide range of crafts – casting of non-ferrous metals, brooches, hairpins and so-called melon beads. Large loom weights even point to a speciality typical of the fleet: sailmaking. This fits two gravestones naming velarii (sailmakers). In 2016, geophysical survey revealed a large stone building of at least 26 × 40 metres in the centre, interpreted as the principia (headquarters building).

Like any long-lived garrison, the fort was surrounded by a vicus – a civilian settlement on the three sides facing away from the river, with strip houses, pottery kilns and a mansio (travellers' rest) in front of the west gate. The cemeteries lay mainly along the northern road leading toward the CCAA.

Around the later 3rd century, finds of tile stamps, inscriptions and everything else attributable to the Classis Germanica come to an end. The late-antique – though uncertain and much-disputed – Historia Augusta records a violent end through a Frankish attack. Archaeology has not yet confirmed this, so it remains open whether the camp was deliberately abandoned or destroyed.