The fortress was originally constructed by Gustav Vasa in 1544 to defend Stockholm against shipborne attacks from the east, but most of the current structure dates from 1833-1863. Russian prisoners-of-war were used in part to build the fortress[citation needed]. The stretch of water below the building was formerly the main sea route to Stockholm. Thus, the fortress was strategically situated to defend the city from naval attacks. It was attacked by the Danes in 1612 and the Russian navy in 1719. Since the mid 19th century, its military importance has ceased. Today, it is home to the Swedish National Museum of Coastal Defence.