[4][7] The fort was later known as Castle William and required incoming ships to recognize the fortification and would fire at them if they didn't offer recognition by raising their flag.

[9][10] After the end of the King Philip's War the fortress was a site of internment of captured Native Americans who were shipped out to be sold into the Atlantic slave trade markets.

[10] Some people who worked at the fort included Thomas Beecher (ancestor of Henry Ward Beecher[citation needed]), a Castle officer; Captain Nicholas Simpkins, a first commander;[12] Lt Edward Gibbons, a first commander; and Roger Clapp, who served for several decades as an officer.

[9] In 1701, Colonel Wolfgang William Romer, the chief military engineer for North America, came to Boston to fortify the harbor.

Major Pelham Winslow of the prominent loyalist town of Marshfield, Massachusetts was the Commander of Castle William for a time during the Revolution.

The island is also the site of a monument to Donald McKay, the builder of the famous clipper ships Flying Cloud and Sovereign of the Seas.

[28][29] Edgar Allan Poe learned of the legend while serving on Castle Island in the Army, and his short story "The Cask of Amontillado" is said to be based on it.

Interpretive programs are conducted by the Castle Island Association in conjunction with the Massachusetts Department of Conservation and Recreation.