Markeli (Bulgarian: Маркели; Greek: Μαρκέλλαι, 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 mi) from the modern town of Karnobat. 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.