Citadel Prins Frederik, Former fortification site in Sawah Besar, Jakarta, Indonesia

Citadel Prins Frederik was a rectangular brick fort with a bastion at each corner, protected by an earth rampart. Its windows doubled as cannon portholes, and a central rectangular tower with clocks, windows and doors on each side served as a lookout. It stood in Wilhelmina Park between the two branches of the Ciliwung River.

The site began as a tavern built before 1669, rebuilt in 1723 by sergeant-major Herman van Baijen as a country house and used from 1743 to 1820 as the Outer Hospital outside Batavia's walls. In 1837 Governor-General Johannes van den Bosch ordered a fort here as part of a defence line in Kebon Sirih, Menteng; Colonel Carel van der Wyck designed it and Captain Lucius Gerhard Johan George Schonermarck built it. Prince Willem Frederik Hendrik laid the first stone and later opened it. It was demolished around 1961 for the Istiqlal Mosque.

Locals called the fort Gedung Tanah, the Ground Building, a name that outlasted the Dutch garrison. It stood inside Wilhelmina Park, a 9.32-hectare garden laid out in 1834 that served as a vegetable plot for Dutch officers and a gathering spot for Company officials. The park also held the Atjeh Monument to Dutch soldiers killed in the Aceh War, removed in 1961.

The fort stood in Wilhelmina Park in Weltevreden, today's Sawah Besar, between the two branches of the Ciliwung River. The park, in front of Jakarta Cathedral, was demolished around 1961 to make way for the Istiqlal Mosque, which now occupies the site.

Local people called the fort Gedung Tanah, the Ground Building, a name that outlasted Dutch rule. The defences were never tested: no enemy attacked Batavia until 1942, when the citadel was already obsolete. Its site is now occupied by the Istiqlal Mosque.