Deep Freeze is the 2nd upgrade of Path 2 for the Ice Tower. It costs $300 on Easy, $350 on Medium, $380 on Hard and $420 on Impoppable. The tower freezes 2 layers of bloons instead of 1 with this upgrade. Despite the description, the tower does not actually do 2 layers of damage. Instead, the description means that the frozen status "soaks through" to the next layer (e.g. a frozen blue bloon will stay frozen even after being popped down to a red bloon.)
Unless there are other non-Ice towers around, Deep Freeze is useless.