Battery Calwell was completed in 1905 and was equipped with four 6-inch guns. These guns were removed in 1917 for shipment to Europe during World War I.
Fort Flagler was established on the north end of Marrowstone Island in 1897. Along with Forts Casey and Worden, Fort Flagler helped defend Admiralty Inlet, the vital passage to the Bremerton Naval Shipyard and ports in Seattle & Tacoma.
Today Fort Flagler State Park maintains the same borders as the original fort, with hiking trails, beaches, camping, museum, 5 vacation houses, retreat center and 10 historic gun batteries open for public exploration.