It was the site of two notable medieval battles between Byzantines and Bulgarians, the Battle of Marcellae of 756 and the Battle of Marcellae of 792.
Markeli acquired its strategic importance in the late 7th century, when the establishment of the First Bulgarian Empire and its expansion turned it into a vital frontier stronghold just south of the Balkan Mountains.
Constantine V's army advanced into Thrace and was engaged by Bulgarian forces at Markeli, which at the time was a border fortress.
The Byzantines came out victorious in the skirmish, The second battle of Marcellae came during a long period of aggression between Byzantium and Bulgaria, with Bulgarian ruler Kardam and Byzantine Emperor Constantine VI each invading foreign territory in the past few years.
The church has only been slightly reconstructed since then.
, Markellai; Latin: Marcellae) was a medieval Byzantine and Bulgarian frontier stronghold, the ruins of which are located in Karnobat Municipality, Burgas Province, southeastern Bulgaria.
Dating to Late Antiquity, the castle lay some 7.5 kilometres (4.7 miles) from the modern town of Karnobat.
A well tower was positioned in the vicinity of the river in order to control access to the fortress and provide water for the defenders.
An ancient bridge crossed the river near the tower, and an underground passage served as another way for the inhabitants to enter or leave Markeli.
The church's dimensions were 30.5 by 17 m (100 by 56 ft).