It was separated from the north-east by a deep moat. The castle used to be the strongest episcopal stronghold in Warmia. Bear in mind that in 1516-1521 Nicolaus Copernicus lived here, and in 1520 he commanded a successful deffernce of the stronghold. In 1945, the castle became the official seat of the Masurian Museum which today is called the Museum of Warmia and Masuria. The collection is very rich. The largest exhibition hall is a former refectory with a sophisticated crystal vault of 1520. In addition to the exhibition activities, events organized at the castle as part of the Olsztyn Artistic Summer as well as the castle nights and "Sundays at the Museum”. are very popular too.