The Nizwa Fort is a massive castle in Nizwa, Oman. It was built in the 1650s by the second Ya’rubi Imam; Imam Sultan Bin Saif Al Ya'rubi, although its underlying structure goes back to the 12th Century. It is Oman's most visited national monument. The fort was the administrative seat of authority for the presiding Imams and Walis in times of peace and conflict. The main bulk of the fort took about 12 years to complete and was built above an underground stream. The fort is a powerful reminder of the town's significance through turbulent periods in Oman's long history. It was a formidable stronghold against raiding forces that desired Nizwa's abundant natural wealth and its strategic location at the crossroads of vital routes.