A summer residence of the House of Welf in the past, it is now owned by the Marienburg Castle Foundation chaired by former owner Ernst August of Hanover but publicly funded in part by the state of Lower Saxony.

When Prussia annexed Hanover in 1866 after the battle of Langensalza, the royal family emigrated to Gmunden in Austria, leaving the castle uninhabited for almost 80 years.

Consequently, Marienburg is well preserved, as few renovation occurred until the, no longer royal, family moved back in late 1945, decades after the German revolution and proclamation of the republic at the end of World War I.

[5] Ernest Augustus and his wife Viktoria Luise moved to Marienburg from Blankenburg Castle, just a few days before it became part of the Soviet occupation zone at the end of World War II.

Parts of it were open to the public, such as the castle museum, the restaurant and the chapel, and could be booked as a location for events such as weddings, receptions and concerts.