Fort built in defense of New York City just before the War of 1812. Castle Clinton wasn't originally on the island of Manhattan as it is now; it was built on a small island to Manhattan's south and was connected to the mainland by way of a 300-foot causeway. The land between was slowly filled in, completely surrounding the fort by 1855 and creating Battery Park.
The fort's name was changed to Castle Garden in 1824 and stayed that way until 1855. During this time the fort hosted a promenade on the top of its walls with understandably spectacular views of the Hudson River. It became New York City's premier cultural institution in the 1840's when a roof was added, converting the building into a large concert hall. Its life as a hotspot was limited, however; the building was converted yet again in 1855 to serve as an immigrant processing facility. Ellis Island took over those duties in 1892 after Castle Clinton had processed over 8 million people. The building was remodeled again in 1896, this time to serve as the New York Aquarium. It served in this capacity until the aquarium left Manhattan for Coney Island in 1941. The fort was closed in 1941, was almost demolished in 1942 and was eventually designated a national monument in 1946. Castle Clinton now serves as a monument, a museum and occasionally as a cultural event venue.
Sources:
http://www.nyc-arts.org/nyc-arts/name/name_by_borough/manhattan/lower3c/cclinton.html
http://www.nps.gov/cacl/
http://www.lowermanhattan.info/history/didyouknow/castle_clinton_83528.asp