After 1398, the Teutonic Knights were no longer able to reconquer the castle. After the Battle of Grunwald, Kaunas Castle lost its strategic military importance and was used as a residence. The castle served administrative purposes after the death of Vytautas the Great. Sigismund Augustus gave this castle to his wife Barbara Radziwill in 1549. During the 16th century, the castle was strengthened and adapted to new defensive purposes by the construction of an artillery bastion near the round tower. The diameter of the bastion was about and the height of the bastion's walls was about ; the wall worked in conjunction with a defensive trench. At the bottom of the bastion a firing gallery was installed, which was linked with the tower.