On August 20th, 1775, Lt. Col. Hugo O'Conor, an Irishman serving in the Spanish Army, founded a fort in what is now downtown Tucson. With the exception of a small Spanish chapel across the Santa Cruz River at the foot of “A" Mountain, this was the first European structure in Tucson. By the 1780s, when the 11-acre fort was completed, it consisted of 10-foot-high adobe walls and two corner towers, each 20 feet high. This fort marked the northwestern edge of the Spanish frontier in Arizona. The northeast corner of the original Presidio has been reconstructed and is known as the Presidio Museum; it is open to the public Wednesdays through Sundays 11 a.m. to 4 p.m., and Tuesdays between January and April. The Museum offers docent tours and many living history events and activities throughout the year.