NEW YORK -- The courtyard of Castle Williams on Governors Island was sultry, the sky threatened rain. Mike Shaver, supervisory park ranger for the National Park Service, turned his key in a heavy metal door and swung it open. Inside, it was cool. The eight-foot-thick sandstone walls of the 200-year-old fort acted like a cave, the ranger said, keeping the interior at a more-or-less constant temperature all year long.

In front of us was one of the fort’s 39 large, vaulted casemates, designed to hold two guns each and to guard New York harbor from enemy invasion. In 1807-1811 when the fort was built, that would have been the British.

The fort, with several landmark accolades that include the National Register of Historic Places, was named for its designer, Lt. Col. Jonathan Williams, a great-nephew of Benjamin Franklin, who shared some of his great-uncle’s scientific and engineering genius. On May 26, it reopened to the public after a three-year, $6.1 million renovation.

Castle Williams and Fort Jay, a slightly older fort, are the core of the 22-acre Governors Island National Monument on 172-acre Governors Island.

Previously forts had been star shaped, like Fort Jay. Williams, the first head of the Corps of Engineers and the first superintendent of West Point, figured out that a round fort on the western flank of Governors Island, facing New York harbor, would allow coverage in all directions. His round forts became prototypes for coastal fortifications.

“One of the reasons why circular towers didn’t work in the old days was that if you penetrated them, they would collapse,” said Shaver. “Williams built arches into the casemates so that in the event that you were lucky enough to go through seven or eight feet of sandstone and breach the walls, the building wouldn’t collapse.”

Nevertheless, Williams’ supervisors were dubious. “His superiors were old fellows left over from the Revolution,” said Shaver. “They were always fighting the last war. They had some doubts about [Williams’ design]. To prove his point, he had two Navy ships fire on the fort from about 400 yards away. All they succeeded in doing was to knock a cannon off its carriage. Williams sent the Secretary of War a newspaper clipping that said the barrage had destroyed a cannon. At the bottom of the clipping, he wrote, ’It didn’t destroy a cannon! It knocked off a cap plate and lock screw and knocked it off its carriage. It’s still serviceable.’ ”

Castle Williams has 78 embrasures (openings) for guns positioned on three tiers. Williams designed the casemates with high ceilings and openings between them in order to help dissipate the smoke from firing the guns.

Telescopes mounted outside Castle Williams that are trained on Castle Clinton in nearby Battery Park and on Fort Gibson on Ellis Island show how these and several other forts positioned according to Williams’ plan provided an impregnable defense of New York harbor. All were built prior to the War of 1812 when the threat of war with the British was imminent. The plan worked. The British burned down Washington, D.C. and attacked Fort McHenry in Baltimore but they never attacked New York City.

A model of the fort as it looked originally is in the middle of the Castle Williams courtyard. It was a handsome building, with outdoor walkways flanking the imposing arches of the casemates. Large murals show key dates in the fort’s history as it evolved from its original purpose — defense of New York harbor — to a prison for Confederate soldiers and Union deserters during the Civil War and later, during the 20th century, to a prison for the U.S. Army that became known as the “Eastern Alcatraz.”

Despite the beauty of Governors Island, Civil War imprisonment at Castle Williams was grim. Dysentery, pneumonia and typhoid were rampant. A sign in one of the exhibits quotes Confederate officer Thomas Sparrow, who noted on Sept. 30, 1861 that of 615 men imprisoned in Castle Williams, more than a hundred were ill. Almost every day, someone died.

By the 1890s, the Army began to use Castle Williams as a military prison, one of 12 located in various parts of the country. Eventually, all Army prisoners were dispatched to one of three places — Alcatraz in San Francisco, Fort Leavenworth in Kansas or Castle Williams.

“In 1947, the Army gutted the entire building, took out the old, oak floors from 1812 and poured in reinforced concrete floors and put in the jail cells that you see,” Shaver said, as he led a tour of the second and third tiers of the three-story structure. The solitary confinement cells had walls of solid metal. There are protrusions in the metal from the prisoners banging on the walls.

The tour leads via a spiral sandstone and brick stairway to the roof, where a Civil War Rodman cannon, once capable of shooting three miles, is positioned facing Manhattan. The panoramic view also includes New Jersey and Brooklyn. “You can see how strategically this fort was positioned,” said Shaver as he looked over a parapet.

The U.S. Army left Governors Island in 1966 and it was taken over by the U.S. Coast Guard, which departed in 1997. During the Coast Guard’s tenure, Castle Williams was sometimes used as a community center and sometimes as a grounds-keeping shop. The roof deteriorated and water seeped into the walls. Federal money was used to remove lead paint, asbestos and other hazardous materials, to replace the electrical wiring and the windows and to repair the stonework and the bricks. It also financed exhibits in the courtyard and on the first floor that tell the history of Castle Williams culminating with a Presidential Proclamation in January 2001 that created the Governors Island National Monument. The entire island was turned over to New York State for public use in 2003, and subsequently, in 2010, to New York City.

Most of Governors Island is a park dotted with buildings formerly used by the U.S. Army and the U.S. Coast Guard. Former officers’ quarters, barracks and recreational and administrative buildings are arranged around lawns shaded by century-old trees and expansive parade grounds that now are used for art exhibits, concerts, festivals and picnicking. The Admiral’s House, the site of a historic meeting in 1988 between U.S. President Ronald Reagan and Soviet leader Mikhail Gorbachev, is open to visitors.

Bicycle rentals make it easy to get around the island, and for those who don’t ride bicycles, a free tram runs every 10 minutes between Pier 101 on the east side of the island and Picnic Point on its breezy southwestern shore.

Governors Island embodies the vision expressed in the Bible: “They will beat their swords into plowshares and their spears into pruning hooks.” In a place once dedicated to warfare, now peace and tranquility reign.