Here, in the sun-swept, mountainous desert of Texas's westernmost corner, is El Paso, the state's fourth-largest city. Built between two mountain ranges on the shores of the Rio Grande, the city is an urban history book, with chapters dedicated to Spanish conquistadors, ancient highways, gunfighters, border disputes, and modern sprawl.El Paso's rich history is a result of its geography. The Franklin Mountains, which now border the downtown area and occupy the city's heart, offered natural defense for the American Indians who inhabited the area for more than 10 millennia; the Rio Grande offered water. As the mountains slope into a vast canyon, the Spanish explorers who first crossed the Rio Grande in the 16th century saw it as an ideal north-south route, one that soon became known as the "Camino Real" (or "King's Highway") and served as a principal trade route for nearly 300 years.With the 17th century came an influx of Catholic missionaries, a group that established numerous missions that survive today. But Spain saw its grip weaken, and a Mexican flag flew over El Paso when independence was established in 1821. This era was short-lived, as Mexico ceded the land north of the Rio Grande to the United States following the Mexican-American War (1846-48). After the railroad arrived in 1881, El Paso earned the nickname "Sin City," thanks to the saloons, brothels, and casinos that lined every major street. A day or two of exploration is worthwhile; take the time to wander downtown, enjoy a Tex-Mex meal, and gain a better understanding of what a border town is all about.