A fortress was built at the location around 1380 by Bo Jonsson Grip, and belonged to his family until the confiscation of mansions and castles by King Gustav I in 1526. The King tore it down, and built a fortified castle with circular corner towers and a wall, for defensive purposes. Of the original medieval fortress, only the façade of a wall remains.

http://en.wikipedia.org/wiki/Gripsholm_Castle