Kutang San Diego (Fort San Diego) is a really small fort built by the Franciscans in in the town proper of Gumaca, Quezon in the latter part of the 18th century. This fort, equipped with at least one canon (shown below), is meant to repel sea pirates from Lamon Bay that often plundered coastal towns in the Philippines during that era. But given the size of the fort, I’m pretty sure that marauders can easily overpower this fort with sheer number.
The fort, built out of adobe and with a tiled roof, was named after St. Didacus of AlcalÃ¡ (Spanish: San Diego de Alcala), a Franciscan and the town’s patron saint. In fact, Gumaca’s resident church is the Cathedral of San Diego de Alcala. To the west of the fort lies the Gumaca Baywalk.
Anyway, you should check out this WaypointsDotPH narrative by Gerry B. Lontok. It shows you exactly how to get there since there are too many places in Gumaca named San Diego and the Fort has been called a park, a fortress, and a watchtower. The narrative also contains several more pictures of this monument recognized by the National Historical Institute.