Built in 1781, Fort William is situated on the banks of the Hooghly River
and named after King William III of England. It stretches from the Western flank of the Red Road and is the military headquarters of the eastern command. The fort is a brick-and-mortar structure constructed in the shape of an irregular octagon with three sides facing the river and five other sides towards the Maidan
. It occupies 5 sq km area and has six gates.
Swimming pool, boxing stadium, firing range and St Peter's Church (built in 1928) are within the fort. There is also a museum housing traditional arms, swords, muskets and machine guns. The Arsenal inside the fort is worth visiting, but a prior permission is required from the Commanding Officer.