The Castle of the Livonian order, which houses Ventspils Museum since September 2001, is the oldest preserved building in Ventspils, built in the second half of the 13th century (first mentioned in 1290).
The castle was built as a convent-house type building with an inner yard, and can be regarded as the oldest medieval fortress in Latvia to have retained its original form virtually unchanged.

In its turn, the castle chapel is the oldest preserved church building in the region of Kurzeme. In the 13th–16th century the castle was used by the brothers of the Livonian Order, later there was an Evangelical Lutheran church in the castle (1706–1835), a Russian Orthodox church (1845–1901) and also a prison (1832–1959). The castle housed various administrative institutions and army units. The Soviet garrison left the castle in 1983. In 1997 the castle fa?ades were restored and the area around it improved. In the course of study and restoration work, unique 15th–17th century mural fragments were discovered under several layers of plaster, and much evidence concerning the castle and its inhabitants was obtained during archaeological excavation.