The remains of the 16th-century castle that gave Oulu (in Swedish, Uleåborg — "the castle on the Ule") its name. The Swedish Crown built the castle around 1590 to defend the eastern frontier against Russia; it was destroyed by a lightning strike in 1793 and never rebuilt.
On the small island where the castle stood, a 19th-century observation tower now houses Café Linnankahvila, a popular summer cafe with a terrace looking out at the river and rapids. A short walk over the bridge from the market square.