After the construction of the stronghold Bohus in 1308 by King Haakon V, the castle on Ragnhildsholmen started to lose its importance as a royal seat. It is not mentioned after 1320. In the later Middle Ages the town's importance further declined. It burned down in 1612, and was afterwards moved to a location near Bohus and renamed Kungälv. Bohuslän continued to belong to Norway until it was ceded to Sweden in the Treaty of Roskilde in 1658. Archaeological excavations began in the late 19th century at Ragnhildsholmen and the monastery of Kastelle kloster site and continue to the present day. Excavation results indicate that major construction works were carried out by the middle of the 13th century. However, there is archaeological evidence for a royal estate slightly north of the city, dating back to the Viking Era. During excavations by Swedish archaeologist Wilhelm Berg (1891–1892), the remains of the monastery were discovered. The principal excavations of the monastery were during 1953 to 1954 and in 1958 archaeological excavations were carried out of a medieval cemetery area in the ancient city. Several excavations in different places within the old city area were carried out between the years 1985-1994.