The Presidio Reál de San Diego, or Presidio, is the first permanent European structure built in what is today the State of California. Construction began in 1769 by soldiers and priests arriving from New Spain (Mexico), though construction was near-continuous over the next several decades.
The site was constructed just above the settlement of Cosoy, where native people (today called the Kumeyaay) lived, probably seasonally for thousands of years due to the proximity to both bays and the San Diego River. The site was built as one of many fortifications along the California coast in an effort to establish settlement rights and keep Russian and British ships from setting up trade.
The Presidio was active from 1769 until the mid-1820s after Spain ceded control of the region to Mexico in 1821. Made from adobe, the site quickly fell into disrepair once people moved out. Today, it is buried underneath the ground just west of the Junípero Serra Museum.