From Olav Haraldsson's Saga:

Summery: In 1016 Olav Haraldsson, also called Olav Digre, settled the village of Borg, later the town of Sarpsborg, east of Folden, the old norse name of the Oslofjord. This area was important for Olav, because the landscape initiated a stronghold against the Danish king Canute the Mighty and the swedish king Olov Skotkonung and his lords. The peninsula by Sarpsfossen, (the Sarpr Waterfalls), was chosen for the king's township settlement because:

1) It was a communication hub. Sea-going traffic meets river traffic on Norway’s largest watercourse, the Glomma River. Shipping routes outside the falls are mentioned several times. Raet, a very firm ground consisting of stone and pebble naterial from the last Ice Age crosses the Glomma at the waterfall, with no waterlogged clay except right down by the riverbanks area. Several ancient roads have been identified here. Land traffic has sought to cross the river near Sarpsfossen – Sannesund, (The Bay of Sands), below and Opsund, (The Bay above), in the north opposite the falls. Up to Sundløkka (The Bay of Meadows) east of the river runs Gatedalen (The Valley with a Road), an old name referring to traffic from the two former municipalities Borge (Borgir) and Skjeberg (Skialbergh). From Raet and the central village in Tune (Tunir) , Torsbekkdalen (The Valley of Thor's Stream) leads to Sannesund.

2) Topography. Up on the ridge of Raet, the ground was dry and even, but the access to the harbor was not as good as in other Norwegian medieval towns, especially during floods. Higher sea levels, however, might have reduced the upstream current. The town’s location almost like an island, with the big waterfall thundering very close, must have created problems but at the same time offered some protection.

3) Relatively fertile and wide villages around the town were favorable for Olav's army's supply, and for the sale and delivery of goods.

4) The mouth of the Glomma is best considered an arm of a fjord. The city's location at the innermost part of the fjord arm has traditionally been favorable. A marketplace nearby is likely.

5) An old central cult site for a larger district was located right by the city (Tune).

6) A large royal estate (Alvim) bordered the city grounds, which was probably crown property and available for city development.

7) Possibly older city structures could have served as a starting point for establishing a market town.

8) The previously poor relationship between Olav Haraldsson and the Swedish king, Olav Skotkonung, was somewhat set aside with the reconciliation between the two kings in Stathelle, Sweden in 1019. The same year, on Candelsmass the 2nd of February 1019, Olav married the daughter of Olov Skotkonung, Astrid, in Borg.

9) In 1026-1027, the Danish king, Canute the Mighty also called Knut the Great, went on a pilgrimage to Rome. Olav took advantage of this break by heading north in Norway. Olav spent the winter in Nidaros, and in the spring, he called for a leidang (levy), then traveled along the coast gathering people for a military campaign against Denmark and the Danish king, probably thinking that attack was the best defense. After Anund Jacob and Olav had been raiding the Danish islands and Skåne for a while, Knut arrived from England with a large fleet, with many discontented Norwegians on board. It ended with Olav being trapped with his ships in the Baltic Sea, which he then left in Kalmarsund, and he set off on horseback with his men through Småland and Western Götaland into Norwegian Båhuslen, and didn’t stop until he rode through his defensive rampart in Borg.

10) The winter of 1027 to 1028 Olav spent his last time in Borg. Even though he could feel somewhat safe there, he wasn’t enjoying himself that winter. Several of the chieftains had left him. Many of those who were still around him were unreliable. He kept some in Borg because he wanted to have them under control. This included Kalv Arneson and all his brothers. Rumors were flying about King Knut’s plans and about how one by one the chieftains were switching to him, allegedly through bribes. In addition, the king was suffering from shortages of food and income. It was actually not easy for him to provide for the fairly large force he had with him in Borg. The stay had probably not been properly planned. The levy and other revenues from most of the country failed completely.

11) Early in 1028, he was almost forced to move inland in Østlandet and stay there. Thus, he left his city for good. In the summer, while Olav was still wandering around Østlandet, Knut the Mighty arrived and was hailed as King of Norway at one assembly after another. When Knut the Mighty held assemblies in Borg and several other places in the summer of 1028, and was hailed there as King of Norway, it marked the end of the prosperous period in the city's history associated with the life of Olav Haraldsson.