It is not only underground that the secrets of the salt mine can be discovered. Not far from the mine stands the castle. It used to be the residence of the royal administrator ( żupnik ) of the salt mines. A defence structure was built in this place at the time of the location of the town in the 13th century. During the reign of King Kazimierz Wielki the castle was extended and included in the defence system of the town. Today the gothic building fortified with a wall, a tower, and outbuildings houses part of the exhibition of the Salt Mine Museum . In it visitors can discover the history of Wieliczka and its salt mine and see a most impressive collection of glass, silver, wooden, and ceramic salt-cellars from various periods.