The first work on the royal fortress on Oslo’s small Akershus Peninsula had started around the late 1290s under King Håkon V, replacing Tønsberg as one of the two most important Norwegian castles of the period (the other being Båhus).

It had been constructed in response to an earlier attack on Oslo by the Norwegian nobleman Earl Alv Erlingsson of Sarpsborg.

The fortress is first used in battle in 1308, when it is besieged by the Swedish duke Erik of Södermanland, who later in the same year wins the Swedish throne.

The immediate proximity of the sea is a key feature, for naval power is a vital military force as the majority of Norwegian commerce in this period is by sea.

The fortress is strategically important for the capital, and therefore, Norway as well.

Whoever rules Akershus fortress rules Norway.