The fort of Sultan bin Sayf al-Yaarubi is situated in the historic and commercial district of Nizwa, the old capital of Oman. The fort was built between 1649 and 1680 to defend the city and the surrounding oasis against the Portuguese. It consists of a single watchtower juxtaposed to a rectangular mass that housed the ruler's living quarters. Its construction was initiated by Sultan Nasir ben Murshid al-Yaarubi and completed by his successor, the Sultan bin Sayf al-Yaarubi, who also built the watchtower.
The circular tower is the most prominent part of the fort. Situated at the southeast corner, it dominates the cityscape, rising considerably above the surrounding urban fabric and palm plantation. The tower measures forty-three meters in diameter and twenty-six meters in height. It is built with irregular stone blocks (taken from older forts) covered with plaster and filled with compacted earth up to fourteen meters above ground level. The cannon platform is located above this fill, and is accessed by a staircase that winds through the compacted fill. The staircase itself is broken into several sections by massive wooden doors reinforced with metal spikes, and machicolations were situated in front of each door to discharge hot oil or water onto invaders. The cannon platform holds seven water wells and a prison that is accessed through an opening in the ground. Twelve meters above the platform, a circular balcony (sentry path) circles the fort. It is accessed from the platform through two flights of stairs running parallel to the curved walls. These walls end with crenellations and niches perforated with small openings to allow those within the fort to fire on the outside.
The tower itself is accessed via a small arched doorway situated in the northern side of the fort. This doorway is elevated and separated from the street by a ditch that is occasionally bridged with a wooden beam. The fort has served as a palace, a prison, and a seat for the government until the capital of Oman was moved to Muscat in 1780 by Sultan Said bin Ahmad bu Said. Considered a major element of Omani history, the fort was renovated in 1990 by the Ministry of National Heritage and Culture.
Al-Qila' wa al-Husun fi Oman. Oman: Maktab Naib Rais al-Wizara li-Shu'un Majlis al-Wizara, 1994.