In June 1855, 30 Latter-day Saints made the 35-day trek to Las Vegas Valley from Salt Lake City to establish the first permanent settlement in southern Nevada.
The Mormons built a "fort" to live in, 150 feet square, on the creek flowing out of Big Springs, thus becoming the first non-Native residents of Las Vegas Valley.
They didn't really need protection against the long-time Paiute residents of the valley; the fort was more to keep their livestock in than the indigenous population out.
The Mormons abandoned the fort and their Las Vegas mission in 1858. The structure was dismantled, piece by piece, by subsequent settlers, until only a small remnant lived on; it served as a storage shed on the original Las Vegas Ranch in the late 19th century before being bought by the town-founding railroad in 1904.
The railroad leased the shed to various tenants in the early 20th century, including the Bureau of Reclamation, which used it as a concrete-testing lab for Hoover Dam. The railroad sold it to the Elks in the fort's centennial year, 1955; the Elks preserved the shed for posterity. The remnant was bought by the city in 1971. A number of preservation efforts kept the Old Fort from going the way of most Las Vegas history. It became a 2.8-acre state historic park in 1990.
Today, the Old Mormon Fort is the oldest original building in Las Vegas -- at the exact spot where Las Vegas began, 500 E. Washington Street.