This ancient Roman castrum was once the headquarters of the commander of the Roman army (praetorium) and was used as headquarters by Emperor Trajan himself during the winters of 105 and 106 A.D. The vestiges preserved show the importance of this construction erected on the left bank of the Danube in Roman times. It was fortified by the Byzantins in the 5th century. The only remains of the old rectangular (136.5 x 123.5 m) castrum are the foundations and the vestiges of the thermae. Nearby, other vestiges of Byzantine and medieval origin, i.e. a medieval church with Roman-style buttresses and a 13th century fortress, have also been preserved to this day.