Around 1180 a defense tower was built on the site where Kalmar Castle now stands. The intention was to protect the area from attacks by pirates and other enemies. By the beginning of the 1200s, Kalmar was established as a city.
During the reign of King Magnus "Barnlock", additional towers and a ring wall were built on the site. The Guard Tower to west contained the original main entrance. At the time the fortress was the most advanced of its kind in Sweden.
For a long period the castle was a powerful defensive compound, primarily through its strategically important location. The Swedish-Danish border was not far to the south. Scania, Blekinge and Halland were Danish territories until 1658.
Arguably, the most significant political event in Kalmar during the Middle Ages was the formation of the Kalmar Union in 1397 in which Sweden, Norway and Denmark were members. Queen Margareta was the mastermind behind the union which unified the countries, through a common monarch and common foreign policy. The union also worked as a counterweight to the German Hanseatic League. With the coronation of Gustav Vasa in Sweden in 1523, the union was formally dissolved.
During the time of the Vasa kings Gustav I, Erik XIV and Johan III, the castle was redesigned into the building it is today. During the 16th century the medieval fortress was made into a Renaissance palace after European fashion. Both Erik XIV and Johan III hired artists and carpenters from Europe to modernize the castle architecture and decorations, according to the preferences of the day. In the King's Chamber one finds one of the oldest preserved pictures of the castle, in intarsia, placed on the door that leads to the privy and the secret passage through which one could escape to other sections of the castle. In the Golden Hall one finds an entirely unique gilded ceiling.
During the Kalmar War, the governor Krister Some gave up the castle to the Danes on 3 August 1611. He was branded a traitor and according to tradition his face is the one engraved on the staircase before the eastern section of the castle. The Danes held the castle until 1613. After the Treaty of Roskilde in 1658, the castle was no longer needed to protect the old border and it lost its strategical location. Thus it was no longer needed for military defense. Royal visits became increasingly rare and the grand halls were used for other purposes. There were prison cells and even a distillery.
Kalmar Castle is a symbol for Kalmar. The castle's history features international politics, court intrigues, fiery sieges and fierce battles. Today the castle is vibrant and lively place where many people visit.