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Plaza in Sonoma, California Sonoma Plaza is the central plaza of Sonoma, California. The plaza, the largest in California, was laid out in 1835 by Mariano Guadalupe Vallejo, founder of Sonoma. Sonoma Plaza is the central plaza of Sonoma, California. The plaza, the largest in California, was laid out in 1835 by Mariano Guadalupe Vallejo, founder of Sonoma.