The Croton Reservoir was truly massive. Built to resemble an ancient Egyptian temple, the walls were 25 feet thick and rose 50 feet above ground level. As the city grew northward, the top of the Croton Reservoir's walls became a popular promenade for New Yorkers who had an unobstructed view for miles in all directions. Edgar Allan Poe made a walk around the top of the walls part of his daily constitutional.
The city's inexorable growth northward and outward doomed the Croton Reservoir. In the late 1880s the city built three aqueduct tunnels that carried fresh water from the Adirondacks to town---they are still in use, and New York City water is still considered the cleanest and finest water available to any big city in the world. It is not even purified.
Obsolete, the Croton Reservoir was pulled down in 1899, one year after the outer boroughs joined the city. In its place rose the famous New York Public Library with its iconic lions---though parts of the reservoir's foundations still underlie the Library.