Mangarin is considered as one of the oldest town in Mindoro and said to be founded by the Augustinian Recollects in the 17th century although I haven’t found yet a document that says all these. The island of Mindoro was one of the most affected with the Muslim slave raidings that many of its town were abandoned and churches left in ruins. The island even became a springboard for these raiders.
The ruins in Mangarin, as what can be found today may point out to be one of the watchtowers that were once used as lookouts for the slave raiders. It’s too small to be a fort and the remnants are the lower parts of this quadrilateral structure only. Heavily overgrown, parts of the watchtower are scattered around.