Fort Lowell was the US Army's largest outpost in southern Arizona during the Apache campaigns of the 1870s and 80s — Geronimo's pursuers were headquartered here. The fort was decommissioned in 1891 and the adobe walls slowly melted back into the desert.
What remains today is a city park with ruins of the commanding officer's quarters and hospital, plus a small free museum in a reconstructed adobe building covering the post's role on the frontier. Free and pleasant for a half-hour wander.