Castle William (fifth) is built on Castle Island.

Designed by Col. Romer, the four-bastioned, brick fort is armed initially with 72 cannons (eventually some 200) and named for William III of England. Destroyed by retreating British troops in 1776, it is initially repaired by Colonial troops under the command of Lt. Paul Revere. Some 400 British prisoners of war are imprisoned here in 1777. The fort is rebuilt under the command of Col. Richard Gridley in 1778. It is replaced by a new building (sixth) in 1799.*