Kalmar castle is situated by the medieval town Kalmar. The royal castle has played a crucial part in Swedish history ever since building began at the end of the 12th century. In the days of Knut Eriksson, a castellum, a round fortification tower was built. In the 1280's Magnus Ladulås had a defence castle built around the castellum, which then became the keep of the castle. The castle took its present day form after a several reconstructions and additions mainly during the 16th century, while the outer ramparts around the castle were built in the beginning of the 17th century.
The castle was severely damaged in the Danish siege in 1611 and in the 18th century it ceased to be an important fortification. Kalmar castle was once regarded, along with the royal castle in Stockholm, Viborg castle and Älvsborg castle, as one of the four most important strongholds that were supporting the country.
Castledfjärden (Castle bay) close to the castle was the medieval harbour in Kalmar, called Kättilen. In the 1930's, extensive excavations took place here, in which several medieval boats were found. In Kalmar there is also a reconstruction of one of these boats, Aluette, built at the Naval Museum in Karlskrona in 1995.