The fortress was improved during the 16th century under the direction of King Gustav I and his sons King Eric XIV and King John III, who turned the medieval fortress into a castle fit for a renaissance king. Kalmar Castle suffered heavy damage during the Kalmar War of 1611-13 and was badly damaged by a fire in 1642. Repairs were begun but from the end of the seventeenth century the castle was allowed to fall into disrepair.