Fort Worden Historical State Park is located in Port Townsend, Washington, on 433 acres (175 hectares) originally known as Fort Worden, a United States Army Coast Artillery Corps base constructed to protect Puget Sound from invasion by sea. Fort Worden was named after U.S. Navy Rear Admiral John Lorimer Worden, commander of USS Monitor during the famous Battle of Hampton Roads during the American Civil War.

Constructed between 1898 and 1920, Fort Worden was one of the largest Endicott Period (1890–1910) forts to be built and a rare example of a post built according to the precepts of the Endicott Board on land not already occupied by an existing fortification. It was located within sight of a potential (if unlikely) enemy fortification, a British Royal Navy installation on Vancouver Island in Canada. The fort was designated a National Historic Landmark in 1976.

Fort Worden was an active United States Army base from 1902 to 1953. Most of it was purchased by the Port of Port Townsend in 1956 and sold to the State of Washington in 1957 to house a juvenile detention facility (the Port retained ownership of the beach from the entrance of the Fort to approximately the pier). In 1971, use was transferred to the Washington State Parks and Recreation Commission and Fort Worden State Park was opened in 1973.

In the 1890s, Admiralty Inlet was considered strategic to the defense of Puget Sound in that three forts—Fort Worden, Fort Flagler, and Fort Casey—were built at the entrance with their powerful artillery creating a "Triangle of Fire" to thwart any invasion attempt by sea. Fort Worden, on the Quimper Peninsula, at the extreme northeastern tip of the Olympic Peninsula, sits on a bluff near Port Townsend, anchoring the northwest side of the triangle. The three posts were designed to prevent a hostile fleet from reaching such targets as the Puget Sound Naval Shipyard and the cities of Seattle, Tacoma and Everett.

The forts never fired a hostile shot, and many of the guns were removed during World War I for use in Europe; all remaining now-obsolete guns were scrapped during World War II. Subsequently, Fort Worden was used for training a variety of military personnel and for other defense purposes.

The oldest building on the post is Alexander's Castle, a brick residence built in 1883 by Reverend John Barrow Alexander (1850–1930) which pre-dates the military presence. A British citizen, Alexander was the rector of St. Paul's Episcopal Church in Port Townsend from 1882 to 1886, as well as Queen Victoria's British Vice Consul in Port Townsend and Tacoma from 1884 to 1897. The building was acquired by the government in 1897.

The construction of Fort Worden began in 1897, continuing in one form or another until the base was closed in 1953.

Designed as part of the massive modernization program of U.S. seacoast fortifications initiated by the Endicott Board, construction work on the initial fortifications above Point Wilson were delayed until July 1897. The property was privately owned and the government had to clear title to the land through condemnation proceedings. The Army Corps of Engineers took charge of building the construction dock, warehouses, and a tramway to haul concrete for the gun emplacements from the dock to the mixing plant. All cement for the batteries was imported from Belgium, shipped around Cape Horn and unloaded at Point Wilson. To meet construction needs, the Army laid a pipeline from Port Townsend and pumped water into large storage tanks inside the fort. The arrival of wet winter weather slowed progress on the batteries. It took 200 men almost three years to complete the excavation and concrete work for the gun emplacements.