Founded over 2000 years ago, Xanten (Castra Vetera, Colonia Ulpia Traiana or Tricensimae in Latin) is one of the oldest cities in Germany, and the only German city which name starts with the letter "X".

The area was settled by various Celto-Germanic tribes since at least 2000 B.C.
In 15 B.C., under Emperor Augustus, the Romans built a camp on top of a hill, which the named "Castra Vetera". It was the main base of the Classis germanica (Roman Navy for Germany), which occupied by 8,000 to 10,000 legionaries. Castra Vetera was destroyed during the uprising of the Batavians in 70 C.E. and a second camp was built on Bislicher Insel (i.e. an isle on the Rhine). "Castra Vetera II" became the base camp of Legio VI Victrix ("Victorious").

The Roman history of Xanten began with the arrivl of the first legions in the year 13/12 B.C. The Lower Rhine region was sparsely populated arount that time, but from then on at least one legion was stationed in the Xanten area. In the 1st century after Christ, the biggest known legionary base of antiquity was established here.

The security and the economic opportunities brought by the Roman camp resulted in the development of a settlement nearby. In 110, it had reached a population of 10,000 to 15,000 inhabitants (including many former legionaries), and was given the rights of a Colonia by the Emperor Marcus Ulpius Traianus, who renamed the city "Colonia Ulpia Traiana". It became the second most importand commercial post in the province of Germania Inferior, only beaten by Colonia Agrippinensis (modern Cologne).

During its height, it was home to more than ten thousand people whose history can be reconstructed from the traces they left in the ground.