Chervien

An Ancient, Late Antique, Medieval, and Ottoman fortress and city south of Ruse in Bulgaria. The large fortified site of Červen (Червен, also rendered Cherven) is located less than 1 km north of the modern settlement of the same name. The fortified settlement peaked in the Late Middle Ages, but there are traces of Thracian settlement and a Roman fortress that was enhanced by Justinian in the 6th century. Within the fortified city (which is, as often, composed of an inner and outer section), at the highest point, was built a fortified feudal castle. Two underground water sources and twelve churches have been discovered; one of these was turned into a mosque after the Ottoman conquest, while another mosque was erected in the upper city. The evidence indicates that Červen served as a major commercial and industrial center and may have specialized in iron mining and working. Its location in the interior of medieval Bulgaria might explain its relative absence from the textual sources, although the Synodikon of the Bulgarian Church mentions the metropolitans of Červen. Three significant monasteries were located in the environs of the city.

https://www.bulgariancastles.com/s-cherven-krepost-cherven/