The four-square Livonian Order castle was founded in 1290 by the Teutonic Knights, making it one of the oldest masonry buildings in Latvia. The fortress passed through Polish, Swedish, and Russian ownership before being converted into a prison in the 19th century and a barracks in the 20th. The current museum was opened after a major restoration in 2001.
The museum covers the castle's many lives, with rooms reconstructed in various periods and a strong section on Ventspils's maritime and trading history. The medieval cellars, the prison cells, and the chapel are the highlights. The courtyard hosts concerts in summer.