Grand as it may seem, it's only what remains of the Saint Paul's Church, built by the Portuguese in 1521 on top of a hillock. The church later fell into disrepair in the hands of the Dutch and British colonists. Today the roof had long since collapsed, and the walls much weathered.
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By the way, if any of you guys are interested in math, i highly recommend this book called
Euclid's Window: The Story of Geometry from Parallel Lines to Hyperspace by Leonard Mlodinow, which i am reading now. Its anti-religion and (believe it or not) anti-multi-level-marketing views are highly entertaining. :)