Frozen Bloons grow sharp icicles that can pop Bloons that touch them.
~ In-game description
Icicles is the Tier 4 upgrade on Path 3 for the Ice Monkey in Bloons Tower Defense 6. With this upgrade, any Bloons frozen by the Ice Monkey to grow icicles. These icicles deal two damage at any Bloon that touches them, and pierce up to 3 bloons. The icicles that form on bloons are considered sharp, allowing the popping of White Bloons and Zebra Bloons but not Lead Bloons. Inheriting the properties of Cryo Cannon, its ice blasts deal 2 layers of damage too.
It costs $1,700 on Easy, $2,000 on Medium, $2,160 on Hard, and $2,400 on Impoppable.
Despite what the patch notes mentioned regarding Cryo Cannon pierce not being affected by the change, Cryo Cannon and its subsequent upgrades actually received a pierce buff from 20 to 30. This also affects the Icicles upgrade.
The damage buffs to Icicles actually make it harder to complete One Ice Monkey Only plus 0-2-0 Village, rather than easier. This is because the blasts deal too much damage for the icicle stockpiles to become retained, not to mention that icicles cannot form if bloons are already popped.