Rahovetz Fortress

The ruins of the fortress of Rahovetz (aka Ryahovates) can be found 3 kilometers to the north-west of Gorna Oriahovitza town and 10 kilometers to the north of Veliko Tarnovo city, on 115 meters above sea level. In III and IV century of A.D. the Roman Emperor Aurelius III was building fortresses for guarding the roads in the country of Moesia (North Bulgaria) - that was the frontier of The Roman Empire. One of these fortresses was called Rahova a bit later. The relief and the great displacement and indentation of the area which were present gave the perfect conditions for building the future fortress. The other reason for choosing this hill was perhaps the near floating Yantra River which is making a great elbow to the east and is dividing in such a way a great area from the Danube plain. In 1199 the fortress Rahovetz still was still used for guarding the road to the capital of The Second Bulgarian Country - Tarnovgrad. 

https://en.wikipedia.org/wiki/Rachovets_(fortress)