The building of Akershus Castle and Fortress (Akershus festning) was commenced in 1299 under king Haakon V and was built in order to protect city from enemy attacks. The vicinity of the sea was of great importance because naval majority of Norwegian commerce in that period was by sea. The fortress was strategically important for the capital, and therefore, Norway as well. Whoever controlled Akershus fortress ruled Norway.
Two rivers that runs from north to the south of the city of Oslo are called Akerselva and Alna river (Alnaelva).
Just a couple of hundred meters from new residential block at Sørenga, at the slopes of Ekeberg hill you can find traces of medieval Oslo (Gamlebyen). In a small park called Middelalderparken lies the ruins of St Mary’s Church. The park was built in 2000 and the small lake surrounding the territory represents the Oslofjord as it was once had been.
The history of the city dates way back to year 1000AD when the medieval town of Oslo was located below the Ekeberg hills, on the east side of the Bjørvika bay in the area today called Gamlebyen (Old Town).