Fort Erfprins is a nineteenth-century fortress in the city of Den Helder. With an area of ??49 ha. Erfprins is the largest fortress in the Netherlands. The fort was designated as a national monument in June 1973 and the fort is part of the Den Helder Defense Line which was constructed from 1807 onwards. At this moment it is still in use as a naval barracks. Every now and then, historical tours are given.