Bjni Fortress, Medieval fortress in Kotayk Province, Armenia.

Bjni Fortress is a medieval castle in Kotayk Province, standing on a rocky outcrop above the Hrazdan River. Natural cliffs protect three sides while fortified walls strengthen the north side, creating a strong defensive position.

Bjni Fortress was built in the 9th–10th century by the Pahlavuni family of the Bagratuni dynasty, and its commander Vasak Holum Pahlavuni reconstructed it. In 1021 Daylami Turkish mercenaries raided the district of Nig; Vasak pursued them to the Kasakh River, killed 300, and was later murdered by a villager as he rested. In 1387–88 Timur besieged and took the fortress, killing Bishop Vanakan.

The fortress's upper section holds the Church of the Holy Cross (Surb Khach), a 9th–10th century domed church whose foundations were uncovered by excavations led by Ighit Gharibyan in 1977–1978. The same campaigns revealed a vaulted palace of two halls, a cistern and a secret passage. In 1648, by order of Catholicos Philipos Aghbaketsi, the church was renovated.

Reaching the fortress requires a moderate hike over uneven ground, with the main entrance located through the northern defensive walls. Wear proper footwear and be prepared for steep pathways and rocky terrain.

The fortress preserves ancient water cisterns built from basalt stone and lime mortar, revealing medieval engineering skill in water storage. These reservoirs were essential for sustaining the garrison during sieges.