The original Fort Stevens consisted of an earthwork, a fort made mainly of dirt, and had gun emplacements and several buildings, all completed in 1865. To protect it from land attack, the fort was surrounded by a moat with a drawbridge. In 1897, as part of a nationwide program to improve coastal and harbor defenses, Fort Stevens started a massive refortification program. When the program was completed, eight concrete batteries - including mortars and long and short range rifles - were constructed. Fort Stevens was one of three forts built at the mouth of the Columbia River. Forts Canby and Columbia were on the Washington State side.

The principal armaments of the new Fort Stevens were 10-inch rifled cannons placed at Batteries Russell, Walker, Lewis and Mishler. Mounted on disappearing carriages, which hid the guns behind concrete and earth walls when not being fired, these guns could fire 617-pound shells at a distance of nine miles. To complement the 10-inch rifles, Fort Stevens was armed with 6-inch rifles at Battery Pratt and 12-inch mortars at Battery Clark. Also, to protect the mine fields at the mouth of the Columbia River, there were two small caliber rapid-fire batteries