Moniak, Medieval fortress in Shiroko pole, Bulgaria.

Moniak is a medieval fortress situated on a hilltop at 586 meters elevation, surrounded by the waters of Studen Kladenets Reservoir. The site spreads across a large area and contains remains of walls, towers, and buildings dating to the medieval period.

Moniak guarded the Iron Gate pass and the Arda valley near present-day Kardzhali, with walls built in the 12th century on the foundations of an older Roman castellum. By 1199 Tsar Kaloyan had taken the area from Byzantium, making it a base for Bulgarian attacks on the Latin Empire. In 1205, crusaders of the Fourth Crusade camped on the Arda below the fortress and elected Henry of Flanders regent of the Latin Empire there; he was later crowned emperor elsewhere.

The name Moniac appears in Old French chronicles of the Fourth Crusade as "Chastel que on appelie Moniac". Local people also called the hill Hisar, and the Turkish name Hisar altı was used for the lower fortress. The site is linked to the legend of Boniface of Montferrat, killed in 1207 in a local ambush.

Reaching the site requires a steep hike that takes about an hour after you reach the end of the vehicle road. Wear sturdy shoes and bring water, as the terrain is demanding.

The site is bounded by vertical rock formations that serve as a natural defensive barrier. These geological features are just as striking as the man-made walls constructed on the hilltop.