Kalundborg, city, northwestern Sjælland (Zealand), Denmark, situated on Kalundborg Fjord. A favourite royal seat in the European Middle Ages (chartered 1485), its castle was a frequent meeting place for the Danehof (national assembly). The castle in later times became a state prison, where Christian II was confined (1549–59) by Christian III; it was razed by the Swedes in 1658. Vor Frue Kirke (church; c. 1170) is a unique landmark, the ground plan being in the form of a Greek cross, with five towers. There is also a historical museum. The city has a sheltered harbour that serves a large power plant. Pop. (2008 est.) city, 16,489; (2005 est.) mun., 48,697.