The Real Fuerza de Nuestra Señora del Pilar — built by Spanish Jesuits in 1635 and rebuilt repeatedly through the 17th and 18th centuries — is the oldest standing piece of European architecture in Mindanao and the symbol of the city. The stone walls, bastions, and a deep dry moat enclose a square compound facing the Basilan Strait, the seaward side originally bristling with cannon to fend off raids from the southern sultanates and from passing Dutch and British squadrons.
The fort now houses the Regional Museum of Natural History, the Marine Life Museum, and an open-air Catholic shrine that draws pilgrims from across the Zamboanga peninsula. The eastern wall — where the famous apparition of Our Lady of the Pillar is said to have appeared — has been kept as a working shrine, with candles and offerings stacked along the parapet at all hours.