The roman castle Sostra is situated 12 km far away from the town of Troyan, near to the village of Lomets. The castle was used as a roman military camp. It was situated on a relatively smooth region in the valley of river Osam. In the excavations are found many coins, vessels, monuments. The most precious find is an ancient Trakian mask dated from V-IV century b.c.
The valley where the castle is located is surrounded by the hills: Trapezitsa (550 m above sea level) to the north, a spur of the Mikrenskite heights (Popina hill and Popinka 500 m above sea level) to the west and Sulashkoto (Kaleto 500 m above sea level) to the east. To the south the valley is opened between the villages of Dalbok Dol and Dobrodan. The castle is build near the place where river Ladana (Lomesha river) and river Osam. The sea level of the castle is 300 m.
The location of the castle is very strategical because it controls the passages from south to Troyan, and the road is passing by very near. The east (main) gate of the castle is facing the countryside "Kalugerskoto", where is the only one way toword the upper stream of river Osam.
The whole valley, in which the castle is located, is about 4 square km. There are known antique objects. The river valley is part of the Troyan Balkan. It is consisted of low hills that lie from the east to the west, cut through by the rivers passing by.