Kalmar is one of Sweden's oldest towns, and was originally a Viking stronghold. It was a center of power for many years during the medieval period as well, and boasts one of the finest castles in Northern Europe.
In the 17th century, the warship Kronan, the largest ship in the world in her time, sank nearby, killing nearly all of the 800 crew members. The ship used gun ports, a technological innovation, and when the vessel turned sharply and heeled over, water rushed in and capsized the ship. An explosion of gunpowder didn't help. The huge treasure recovered recently from the wreck (in an ongoing salvage operation) is on display at a local museum.