Castle Island, the oldest continuously fortified site in British North America, got its name because every fort located here was always informally known as the "Castle." As its strategic location allowed for the protection of Boston harbor from sea attack, in 1634 Governor Dudley of Massachusetts Bay Colony ordered the construction of an earthwork with three cannons.

During the Revolutionary War, the evacuating British destroyed the island's fortifications. It was repaired by troops under the command of a young lieutenant named Paul Revere.

Other notables connected with the island include 19th century American author Edgar Allan Poe, who served here for five months in 1827 under the name of Perry. His story, "The Cask of Amontillado" is believed to have been based on an early Castle Island legend.

Now in its eighth incarnation, the fort consists of a five-pointed granite structure containing over 172,687 linear feet (52,635 linear meters) of hammered stone. Colonel Sylvanus Thayer directed the construction between 1834 and 1851. Nearby Quincy did not provide the stone, however, as many people believe. Local stone was more expensive, so granite was hauled in by boat from quarries on Cape Ann, much further away.

In 1818 large sea serpents were reportedly sighted off Castle Island, throwing the city of Boston into a panic. There's been no such report since, so it's unlikely you'll be bothered by them in the least.