Fortification of Diamond Head began in 1908 with the construction of gun emplacements and an entry tunnel through the north wall of the crater from Fort Ruger known as the Mule Tunnel. Originally, it was 5-feet wide and 7-feet high. Mules were used primarily to pull narrow gauge rail cars loaded with material in and out of the crater and to the various construction points.
The trail to the Diamond Head summit was built in 1908 as part of the US Army Coastal Artillery defense system. The battery emplacements were dispersed for concealment and to insure that a projectile striking one would not thereby endanger a neighbor. (Its name later changed to the Kapahulu Tunnel.) The headquarters of the Harbor Defenses of Honolulu came to Fort Ruger in January 1927.