Oslo was first established in the 11th century in the area around where the Alna River flows into the Oslofjord, due to the strategic location both with regard to transport, trade and military. The place became the seat of a bishop in 1100. By 1300, the population had reached about 3000. Construction of Akershus Fortress started in 1299. At the time, the city was mainly made of wooden buildings, and had six churches, three monasteries and two manors: one for the king and one for the bishop. The city declined during the 15th and 16th century. Following the reformation in 1537, the economic base of the city fell away, and the city was repeatedly struck by fire.