Guillermo Castro (soldier)
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For the footballers, see Guillermo Castro (Salvadoran footballer) and Guillermo Castro (Spanish footballer).
Don Guillermo Castro, born in 1809 in Spanish Alta California, was a Californio ranchero, military officer, local justice of the peace and surveyor who once owned vast land holdings in Alameda County. He is the namesake of the unincorporated community of Castro Valley, California and his land included most of the modern City of Hayward, California.