Kalmarsund

Kalmarsund is the passage between Öland and the Swedish mainland. Medieval Kalmar was a port for iron from Småland, and the sound's herring fisheries attracted foreign ships, including German and Dutch vessels. Islets and shoals sheltered its natural harbour, while a castle and town walls protected the mainland port.

Control of the navigable channel became a military objective during the Kalmar War of 1611–1613. Danish troops built a redoubt on Svinö, opposed by Swedish positions on nearby points and islands. Kalmar was captured in the summer of 1611. Plans subsequently called for stronger defences around the town and sound, although the proposed strengthening of Svinö appears never to have been carried out; by 1726 its redoubt stood in ruins.