Kalmar, Sm?land, Sweden. Kalmar Castle is located near the site of Kalmar's medieval harbour and has played a crucial part in Swedish history since its initial construction as a fortified tower in the 12th century. King Magnus Ladul?s ordered the construction of a fortress around the tower in the 1280s, and it was further expanded in the centuries that followed.The fortress was improved during the 16th century under the direction of King Gustav Vasa and his sons Eric XIV and John III, who turned the medieval fortress into a castle fit for a renaissance king. Today, it is one of Sweden's best preserved renaissance castles.One of the most significant political events to take place at Kalmar Castle was in 1397, when the Kalmar Union was formed - a union of Denmark, Norway and Sweden, organized by Queen Margaret I of Denmark. Modern Sweden emerged from the Kalmar Union of 1397 and the unification of the country by King Gustav Vasa in the 16th century.