Since 1718, when the armed Presidio de Béxar was established to defend the Spanish missions, the military has played a key role in San Antonio's development, and it remains one of the largest employers in town today. The 3,434-acre Fort Sam Houston affords visitors an unusual opportunity to view the city's military past (the first military flight in history took off from the fort's spacious parade grounds) in the context of its military present -- the fort currently hosts the Army Medical Command and the headquarters of the Fifth Army. Most of its historic buildings are still in use and thus off-limits, but three are open to the public. The Fort Sam Houston Museum, 1210 Stanley Rd., Bldg. 123 (tel. 210/221-1886; free admission; Wed-Sun 10am-4pm), details the history of the armed forces in Texas, with a special focus on San Antonio. The U.S. Army Medical Department Museum, 2310 Stanley Rd., Bldg. 1046 (tel. 210/221-6277 or 221-6358; www.ameddgiftshop.com/museum.htm; free admission; Tues-Sat 10am-4pm), displays army medical equipment and American prisoner-of-war memorabilia. The oldest building on the base, the Quadrangle, 1400 E. Grayson St. (no phone; free admission; Mon-Fri 8am-5pm, Sat-Sun noon-5pm), an impressive 1876 limestone structure, is centered on a brick clock tower and encloses a grassy square where peacocks and deer roam freely. The Apache chief Geronimo was held captive here for 40 days in 1886. Free self-guided tour maps of the historic sites are available in all three buildings. Anyone wishing to visit the fort must enter through the Walters Gate (take the Walters St. exit off I-35) and present a driver's license.