BTD Science 101 - of all towers in the game, only the mortar with bloon buster upgrade can strip more than 1 layer of bloon at a time.
~ BTD4 after Round 56
Bloon Buster is the 3rd upgrade for the Mortar Monkey in BTD4, the 3rd upgrade in path 1 in BTD5, and the 2nd upgrade in path 1 in BTD6. It pops 2 layers of Bloons. It is powerful against a group of Bloons. The Mortar Monkey's color changes from green to white (red in BTD4). Groups of Bloon Buster Mortar Monkeys can really thin massive amounts of Bloons. As stated in the Bloons TD 4Pre-Round Comment, this upgrade is the only tower that can strip more than one layer at a time in Bloons TD 4.
In BTD4 and BTD5, this upgrade costs $680 on Easy, $800 on Medium, $865 on Hard and $960 on Impoppable. In BTD6, this is reduced to $550 on Easy, $650 on Medium, $700 on Hard, and $780 on Impoppable.