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In the Santo António fort, from the 16th century.
Also worth a visit is the fort at the northern tip of the island, which contains the church of Nossa Senhora do Baluarte, almost certainly the oldest surviving European building in the Southern Hemisphere, dating back to 1522. Be sure you pay the right attendant, or you will have to pay the entrance fee twice: Ask for an entrance ticket, if you don't get one, don't pay.
Capela de Nossa Senhora do Baluarte
Our Lady of Baluarte from 1522 is one of the oldest European structures in the Southern Hemisphere.