Fort Gorges (pronounced "gorgeous") is a military installation that dates to the Civil War era, when Portland's harbor was still a strategic military target. It was built more or less in the ocean on top of Hog Island Ledge. Its thick granite walls were built to withstand cannon blasts, which means that they're still in good condition to withstand the constant waves of Casco Bay.
Its construction continued during wartime, but by the time of its completion in 1865, wartime advances in artillery had already made its walls obsolete against the largest cannons. According to this history, "a modernization plan was begun in 1869, but funding was cut off in 1876, with the third level of the fort still unfinished." That third level was instead covered over in a mound of sand, to insulate the interior of the fort and its stores of gunpowder against attack. Today, that mound of sod grows wild with small trees and shrubs.
Inside the fort's walls is a large open parade ground, which is remarkably calm and quiet:
Granite staircases are still in good condition and lead up to the second and third levels, where there are dark tunnels that lead into the pitch-black powder magazines. The northern side of the fort, facing the city, houses the remnant woodwork of the officers' quarters. Walk on the rotted-out floors at your own risk.
During World War Two, the Fort was basically used as a military storage unit, and there's a concrete pad in the central field where sea-mines were allegedly stored. In 1960, the military finally decided that it had no further use for it, and they donated it to the City of Portland, which has maintained it as a public park ever since. The military cleaned out everything that wasn't nailed down or made of granite, with one exception: a large, Civil War cannon on the eastern side of the third level, which was apparently too large to move out and melt for scrap. It's still there, pointing towards the harbor, half-overgrown in grass and daisies:
The Fort is wide open to the public, but you need a boat to get there - some water taxis will take you there at high tide, or you can rent a kayak (as we did) and paddle there from the East End boat launch. It is a pretty excellent adventure.