The main road along the Oslofjord runs to Sarpsborg (pop. 12,000), originally founded by St Olav in the 11th century. It is now a busy industrial town (papermaking, cellulose, electrical engineering). Borgarsyssel Open-Air Museum (old houses, pastor's house of 1660, ruined church of St Nicholas, built by King Øystein in the 12th century);
beautiful Kulås Park (Iron Age burial mound).
Sarpsborg, originally known as Borg, has been rebuilt on several occasions. It has grown steadily reaching 50,000 in 2005. Sports are popular with a well-known football and hockey team.