Bloons become brittle and take extra damage while frozen.
~ BTD 6 Description
Embrittlement is the fourth upgrade of Path 1 for the Ice Monkey. It allows the tower to pop 2 layers of bloon and frozen bloons take extra damage from other attacks (+1 damage to be exact), including sharp objects (for a brief time after thawing out). It costs $2,550 on Easy, $3,000 on Medium, $3,240 on Hard, and $3,600 on Impoppable.
This upgrade is the only upgrade of the Ice Monkey where the skin color of the monkey is not a cold color (i.e. greens, blues, purples). Instead, Embrittlement's skin color is red.
"Embrittlement" is an actual word used within the material science study field that describes a decreased toughness of a material, thereby a loss of ductility, in turn rendering the material to become brittle. Objects that suffer from embrittlement become easier to rupture when forced under pressure, similar to what the Ice Monkey's Embrittlement upgrade does to bloons.
Unlike actual embrittlement, lowering the temperature of a material only temporarily renders a material to become brittle, until the temperature returns to the original amount. This is similar to what happens with the Ice Monkey's Embrittlement upgrade, as bloons will only temporarily gain the embrittlement vulnerability including after thawing.