Standalone short: in 1015 a Swedish and a Norwegian king settled who owned the island of Hisingen by casting dice at Kungahälla — a glimpse of how contested the Göta älv mouth once was. Leif + Margareta.
Two kings. One island. And to decide who got it — a pair of dice. [pause] This really happened, right here at the mouth of the Göta älv.
A thousand years ago this rivermouth was the seam where three kingdoms met — Norway, Denmark, and a young Sweden with no coast of its own. The Norwegian king's town of Kungahälla was where they came to bargain over it all.
And the saga says that in the year 1015, the Swedish king Olof and the Norwegian Olav the Holy sat down at Kungahälla, could not agree who owned Hisingen — and settled it the only fair way they could think of. They rolled the dice for an entire island.
That is how uncertain this ground once was — you could win or lose it on a throw. It took the regent Birger jarl, around 1254, to finally secure the rivermouth for Sweden. And to hold a prize like that, you need more than luck. You need a fortress. Its name would be Älvsborg.