Kalmar Castle is located on Slottsholmen Square near Slottsfjärden, where the medieval Kalmar harbour was located. It has played a major role in Swedish history since the end of the 12th century. century, a castle was built. During Knut Eriksson’s time a castle was built, a circular castle tower. V 80. Years 12. century, Magnus Ladulås built a defensive fortress around the castle, which then became the main castle tower. – Kalmar Castle was considered one of the four most important Swedish fortresses that held the country, along with Tre kronor Castle, Viborg and Älvsborg Fortress.