Långholmen is a small island in the centre of the city of Stockholm, which once was the city jail. It is now a park, but still houses the old prison buildings, with a museum inside. You can visit the museum every day between 11am and 4pm. Admission costs 25 SEK, about 3 euros. Sometimes there are guided tours during the tourist season, and you can find information on the island's website, or at the tourist office in the centre of English. Like the vast majority of Swedes, all the guides speak fluent English, and the tour is in English twice a day. The prison was used in the Middle Ages, and gradually grew to become the city's largest prison, with more than 500 cells.