They thought it was a good place to start a mission
They did the same as they did in other missions. They make adobe bricks to create the mission and maybe a few clay pots.
Mission San Diego de Alcalá is a business in California.
Father Serra designed all 21 missions.(:This is the 1st mission he built,he died in 1784.):
Mission San Diego de Alcalá: area - 0.0182115 square kilometres