Markeli, Medieval fortress in Karnobat, Bulgaria.

Markeli is a medieval fortress on Hissar hill near the Mochuritsa River, featuring stone walls reinforced with brick construction across its layout. The remains reveal a complex structure of defensive and residential areas from different periods.

Markeli entered Bulgarian hands in 705, when Justinian II ceded it with the Zagore region. At this frontier stronghold Constantine V defeated Vineh in 756, and in 792 Kardam's Bulgarians routed Constantine VI, seizing his tent, treasury and horses. Nikephoros I set out from Markeli on his fatal 811 campaign, and in 1089 Alexios I Komnenos used it against the Pechenegs and Cumans.

On the high northeastern edge of the fortress, archaeologists found several Christian buildings stacked one above the other. A martyrium of 3.8 by 4 metres was built there at the end of the 4th century; after its partial destruction in the second quarter of the 6th century, a large three-nave basilica replaced it. That basilica was ruined in the 7th or 8th century and replaced in the 9th century by a smaller cross-domed church.

The fortress occupies the western tip of Hisar Heights, with the highest point at 225.5 m. The ramparts and ditches, up to 10 m high and over 3 m deep, make the terrain steep and uneven. The Mochuritsa river below is the main water source; no facilities are on site.

The fortifications were built of crushed stone with bands of brick, likely under Anastasius I (491–518) or Justinian I (527–565). A well tower by the Mochuritsa controlled the river crossing, where an ancient bridge stood, and an underground passage gave a second way in and out.