The history of the city dates back to Roman times. In about 11 BC, the Roman Army appears to have stationed a small unit in what is presently the historical centre of the town. Even earlier, the Army had resettled members of a Germanic tribal group allied with Rome, the Ubii, in Bonn. The Latin name for that settlement, "Bonna", may stem from the original population of this and many other settlements in the area, the Eburoni. The Eburoni were members of a large tribal coalition effectively wiped out during the final phase of Caesar's War in Gaul. After several decades, the Army gave up the small camp linked to the Ubii- settlement. During the 1st century AD, the Army then chose a site to the north of the emerging town in what is now the section of Bonn-Castell to build a large military installation dubbed Castra Bonnensis, i.e., literally, "Fort Bonn". Initially built from wood, the fort was eventually rebuilt in stone. With additions, changes and new construction, the fort remained in use by the Army into the waning days of the Western Roman Empire, possibly the mid-5th century. The structures themselves remained standing well into the Middle Ages, when they were called the Bonnburg. They were used by Frankish kings until they fell in disuse. Eventually, much of the building materials seem to have been re-used in the construction of Bonn's 13th century city wall. The Sterntor (star gate) in the town centre is a reconstruction using the last remnants of the medieval city wall.
To date, Bonn's Roman fort remains the largest fort of its type known from the ancient world, i.e. a fort built for one full-size Imperial Legion and its auxiliaries. The fort covered an area of approximately 250,000 square metres (62 acres). Between its walls it contained a dense grid of streets and a multitude of buildings, ranging from spacious headquarters and large officers' houses to barracks, stables and a military jail. Among the legions stationed in Bonn, the "1st", i.e. the
