Starting in 1769, Spain built a chain of 21 missions across the length of Alta California—from San Diego to Sonoma—as a way of gaining a foothold in the new frontier. California’s mission era ended in 1834, but you can still see the architectural legacy in the state’s red tile roofs, whitewashed walls, arched colonnades, and bell towers. The missions were built approximately 30 miles apart—about a day’s journey by horseback—and all 21 are open to visitors and feature a gift shop and museum.