Castle Gap, a mile-long pass between Castle and King Mountains on the Upton-Crane County line, was an early route for southwestern trails through which passed much of Texas history, including nomadic Indian tribes in prehistoric times, Commanches traveling their war trail into Mexico, Spaniards, stage traffic, trail herd drivers, settlers, and 49ers bound for the California gold fields. The pass was probably first discovered by a white man in 1761 by the Spanish explorer Felipe Rabago y Teran. From 1858 to 1861, Butterfield Overland Mail coaches traveled through the gap on their 24-day journey from St. Louis to San Francisco, stopping at the adobe-walled Castle Gap Station for fresh teams before fording the Pecos River at Horsehead Crossing 12 miles to the southwest. By 1866, the Goodnight-Loving cattle trail was firmly established there, funneling tens of thousands of longhorn cattle to northern markets. In the same period, legend held that a treasure-laden aide of Emperor Maximilian of Mexico, fleeing after the collapse of the regime, buried gold and jewels in the area. Today Castle Gap is peaceful, visited mainly by travelers, occasional treasure hunters, and people searching for ruins of the Butterfield Station and fading coach and wagon ruts.