Admiralty Inlet was considered so strategic to the defense of Puget Sound at the turn of the century that three forts were built at the entrance with huge guns creating a "Triangle of Fire" that could theoretically 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. Fort Casey, on Whidbey Island, sits on Admiralty Head almost directly across Admiralty Inlet from Fort Worden. Fort Flagler anchors the southwest side from a bluff on Marrowstone Island.