Monyak (Bulgarian: Моняк) is a 12th-century medieval fortress in the Rhodope Mountains of southern Bulgaria, mentioned in the chronicles of the Fourth Crusade as Chastel de Moniac in Old French.

[1][2] Monyak is identified with the fortress ruins on a 587 m height rising on the left bank of the river Arda over the modern Studen Kladenets Reservoir.

[1] Its remains include parts of the defensive walls, an octagonal tower and water reservoirs dated from the 12th century, probably during and/or before the Third Crusade, built on the foundations of a significantly older Roman castrum.

The preserved part of the outer fortress wall along the northern side of the hill is more than 270 m long, and its height in places reaches 7–8 m. It is built of stones welded with lime and sand.

So Henry the Regent of the empire, remained in the camp, with a great part of the host, and the remainder went forward to relieve Renier of Trit at Stanimac.