Long time ago, when the place had no name yet, almost half of it was a swampy place. Nipa and big bakhaw trees grew abundantly in the area. During high tide, sea waters rose up through the rivers that surrounded the place. As there were only four families originally living here, fish, crabs and shrimp abound and could easily be caught by the inhabitants.
One day, while a group of young boys were fishing along the river bank, three Spaniards who were riding on a boat passed by. They asked the boys in their own language what the name of the place was. But the boys did not understand what the Spaniard said. So one of the boys shouted “Kastila Buang”! Upon hearing what the boys had answered, the Spaniards thought that the “Kastila Buang” was the name of the place so they repeatedly remarked, “Ah, Kastila buang! Kas buang!”. And from that time on the place was named “Casbu” which derived from the word “Kastila buang” or “Kas buang”.
Casbu was once a part of Bato. It was one of the sitios of Bato. But later on it was separated from Bato and became a small barrio in 1968. It has a total land area of 50.4491 hectares with two sitios, namely; Casbu Ilaya and Casbu Ilawod. It is 1,300 meters away from the poblacion of Guindulman. Casbu has a swampy area bounded by a river with brackish water. So owners of these are had developed them into fishponds up to the present time. Casbu has also a wide irrigated rice lands with a population of 800 inhabitants more or less. Their important products are palay, coconuts and bananas. Mostly, the people are farmers and fishermen while only few are professionals and carpenters.