Castle William was an island fortress in Boston Harbor. It was located on Castle Island that lay a short distance off Dorchester Point at the eastern end of Dorchester Neck. Castle William became a refuge for the British during the early part of the American Revolution. It was destroyed when the British evacuated Boston in March 1776. In 1799, the United States government was considering the construction of a new fortress on Castle Island, and it was at that time that the name of the fort was changed to Fort Independence. Construction on the new fort began in 1801 (see Roberts, Historic Forts, 402-4).

GW crossed over to Castle William during his visit to Boston to see Gov. William Shirley in early 1756 (see Ledger A, 27).