Fort Casey State Park is located on Whidbey Island in Washington state. Admiralty Inlet was considered so strategic to the defense of Puget Sound in the 1890s that three forts, Fort Casey on Whidbey Island, Fort Flagler on Marrowstone Island, and Fort Worden at Port Townsend, were built at the entrance with huge guns creating a "Triangle of Fire." This military strategy was built on the theory that the three fortresses would thwart any invasion attempt by sea. Fort Casey is now a 467-acre (1.89 km2) marine camping park. The Admiralty Head Lighthouse is located in the state park.
Three miles of the Pacific Northwest National Scenic Trail wrap around the park.
Designed as part of the massive modernization program of U.S. seacoast fortifications initiated by the Endicott Board, construction on Fort Casey began in 1897. In 1901, her big guns on disappearing carriages, which could be raised out of their protective emplacements so that the guns were exposed only long enough to fire, became active. However, the fort's batteries became obsolete almost as soon as their construction was completed. The invention of the airplane in 1903, and the subsequent development of military aircraft made the fort vulnerable to air attack. In addition, the development of battleships designed with increasingly accurate weaponry transformed the static strategies of the nineteenth century into the more mobile attack systems of the twentieth century. Most of Fort Casey's guns and mortars were removed and sent to Europe and the Pacific during World War I, where they were mounted on railcars to serve as mobile heavy artillery.
- "Fort Casey State Park". Washington State Parks. Retrieved 2014-08-19.
- Harris, Alfred W. (November 1968). Comment and Discussion. United States Naval Institute Proceedings.
- Powers, William M., PHC USN (June 1968). Comment and Discussion. United States Naval Institute Proceedings.
- Washington State Parks: Fort Casey
- Seattle Pacific University Casey Conference Center history page
- Fort Casey fortifications
- The Coast Defense Study Group
- Fort Worden History