In the Middle Ages, there were important settlements in the area: the fortress "Skafida", "Poros", "Rusokastron" (Battle of Rusokastro), the Baths called Aquae Calidae and used by Byzantine, Bulgarian and Ottoman Emperors; a small fortress called "Pyrgos" was erected where Burgas is today and was most probably used as a watchtower. Under the Byzantine Empire it became an important city on the Black Sea coast. The Bulgarian ruler Krum built the Erkesiya, a -long border wall from the Black Sea (near Gorno Ezerovo) to the Maritsa River.