The foundations of the city were laid when Erik Axelsson Tott founded the castle of Olavinlinna on the island of Kyrönsalmi in 1475. The castle, named after St Olaf, which was initially also known as the "New Castle", Nyslott in Swedish, was built to protect the eastern border between Sweden and Russia. Savonlinna has a population of 34 686 (31.12.2017), and an area of 3,598 km², 1,358 km² of which is water. The population density is 15.48 inhabitants/km². Savonlinna is particularly known for its castle, Olavinlinna, and the Opera Festival held there.