Blagaj Fort locally known as Stjepan grad, is a town-fortress complex near the town of Blagaj, Bosnia and Herzegovina. The old Blagaj Fort (Bona, Stjepan grad) was built on a high, inaccessible karst hill, at an elevation of 310 metres (1,020 ft) above sea level and 266 metres (873 ft) above the source of the river Buna. Blagaj Fort is 275 metres (902 ft) above sea level.
The archaeological material scattered above the slopes of Blagaj hill indicate that settlements existed here during the prehistoric and Roman periods. Remains of fortifications were discovered on each of the summit's highest points: On the north-eastern summit, there are the remains of a Roman or late antique fortification or observation post (specula, burgus) known as Mala gradina, while on the south-eastern summit the contours of a prehistoric hillfort can be discerned. The south-western summit contains the remains of the present day Stjepan grad, a medieval or Ottoman period fort. The shorter sides of the triangle are bordered by a gorge through which a river once flowed, and on the longer and only accessible side the remains of massive ramparts are visible, enclosing a fortified town complex of more than 2 hectares (4.9 acres) in area.
It is possible that this complex consisted of two parts in the early medieval period – the Old Fort (Stjepan grad) and Mala gradina, and that this twin settlement lasted at least until the mid-tenth century. The earliest indirect source in writing on the Zahumlje forts, including Blagaj, is the "Treatise on Peoples" by the Byzantine Emperor and writer Constantine Porphyrogenitus, dating between 948 and 952, in which two forts are referred to – Bona and Hum.
After the 10th century, Blagaj played an important role in the development of Hum or Zahumlje. A major influence on its development was the proximity of a major route linking the Adriatic sea with the Bosnian hinterland via the Neretva valley (“via Narenti”). Turbulent political events, particularly after the tenth century, did not have any essential impact on the economic development of the town besides the occasional ramparts. The prince of Hum, Miroslav, held court in Blagaj (Orbini, II Regno, 350). During his time in the church of SS Cosmos and Damian was built. A plaque with an inscription in Cyrillic, found in 1912 near the ruins of the local manor of Bišća and the locality Vrači records the construction of the church (Vego, 1957, 15). The plaque is now kept in the National Museum of Bosnia and Herzegovina.
By the time of King Tvrtko, Bosnian rulers were issuing charters in Blagaj, and in May 1404 Blagaj became one of the residences of Duke (vojvoda) Sandalj Hranić, and then of count (herzeg) Stjepan Vukčić Kosača, after whom the people named the fort Stjepan grad. The first written reference to it is a peace accord between Duke Sandalj Hranić and the Venetians, from November 1, 1423, issued “in our town of Blagaj”. It was also mentioned in the 15th century charters of the King of Aragon and Naples, Alphonse V.
The Ottomans occupied Blagaj in 1465, and by 1473 references to the kadija of Blagaj already exist. The Ottomans repaired the fort twice: in 1699, when the west tower was repaired, and again in 1827. A garrison was stationed there until 1835, although the fort’s former strategic role had long since been taken over by Mostar.