Presidios, or military forts, were established to assist missions with the assimilation of California into the Spanish empire. Four presidios were built along the California coast during the Spanish occupation. Like the missions, the sites for the presidios were carefully chosen to provide fresh water. Sites were also chosen for defensibility in case of attack. The presidios were evenly distributed between the missions so that soldiers could be quickly deployed in the event of a Native uprising or if they were needed to capture runaways. Soldiers also assisted the colonization effort by exploring, hunting, mail delivery, building and maintaining structures, cultivating crops, and herding, although Natives converts performed the most menial chores.
To provide food for the soldiers, civilian towns (called pueblos) were founded near the presidios.
Rawls, J.J. 2000. California History Online [Web site]. California Historical Society [cited October 22, 2003]. View on-line source.