What is "ustra"

üstra Hannoversche Verkehrsbetriebe AG is the operator of public transport in the city of Hanover, Germany.

From 2003 to 2006, it had outsourced its operations, but officially resumed as a service provider on January 1, 2007.

Ustra is a castle in the eastern Rhodope Mountains in southern Bulgaria. Its ruins lie southwest of the village of Ustren situated on a hill at approximately above sea level.

The fortress was built in the 10th century AD to protect an important trade route. It was taken by the armies of Simeon the Great (893-927), but after his death, it was among the lands given back to Byzantium in return for recognition of the imperial title of the Bulgarian rulers. Between the 12th and 14th centuries, it frequently changed hands between the two empires, but the Byzantines held it most of the time.

The ruins are located from the road between Ustren and Zlatograd. There is a hut of the same name nearby. The height of the surviving walls is .