Sarpsborg is a city and municipality in the county of Østfold, Norway. It was founded by the viking king Olav Haraldsson (Saint Olav) in 1016. At that time it actually was the capitol of Norway.
The name Sarp means waterfall, which was, and still is, an important feature for the industry.
Borg means fort, or fortress, so the name says it all; Sarpsborg was a fort by the waterfall.
Unfortunately most of the old city landed in the river Glomma, after a mudslide between 12-1400. The city was rebuilt at a safer location but was burnt down to the ground by the Swedish invaders in 1567. The whole city evacuated downstream to what we today call Fredrikstad, about 15 km downstream. But Sarpsborg was rebuilt, and the population is steadily growing. During the summer of 2005 it reached 50 000.
Together with Fredrikstad, Sarpsborg forms the fifth largest urban area in Norway. As of April 1st 2006, according to Statistics Norway these two municipalitites have a total population of 121 097 - 50 135 in Sarpsborg and 70 962 in Fredrikstad.