Fort Willem II, Dutch colonial fort in Ungaran, Indonesia

Fort Willem II is a fortification in Ungaran featuring four bastions and thick stone walls that enclose a two-story central building. The structure overlooks both its main entrances and sits along the route connecting Semarang and Surakarta.

Built by the Dutch in 1786 as Fort de Ontmoeting, it guarded the Semarang–Surakarta trade route alongside Fort Herstelling in Salatiga and Fort Veldwachter in Boyolali. In 1826 Kyai Mojo's troops besieged it for two weeks during the Java War. After Diponegoro's capture in 1830 he was held here three days before transfer to Batavia. It later served as a British hospital, a Dutch military camp in 1918–1919 and a Japanese prison.

Local people also call it Benteng Diponegoro, after the prince held here for three days in August 1830 before his transfer to Batavia. The name keeps alive the memory of the Java War leader, whose troops under Kyai Mojo had surrounded the fort for two weeks in 1826 without taking it.

The site currently functions as residential quarters for Indonesian police and their families, limiting public access for visitors. Plan to view the structure from outside or ask permission if you wish to get closer.

The fort's original name, Fort de Ontmoeting, commemorates the meeting of 11 May 1746 between Pakubuwono II and Governor-General Gustaaf van Imhoff, where the transfer of the Mataram capital from Kartosuro to Surakarta was agreed. The name means "the meeting" in Dutch.