The immunity to sharp objects does not extend to explosive damage. Thus, you could combine an Ice Tower with a Bomb Tower, Mortar Tower, Monkey Apprentice, Super Monkey, or simply a Pineapple to freeze bloons in place while destroying them.
The Ice Tower also has upgrades that allow it to pop bloons as they're frozen, slow down bloons even after defrosting, and more.
Other towers also have upgrades that will allow them to pop frozen bloons, such as the Sniper Monkey, the Boomerang Thrower, and the Dartling Gun. Additionally, the Monkey Village can be upgraded to permit all towers within its range to pop any type of bloon, including frozen bloons.
In short, Ice Towers are capable of holding bloons in place for other towers to shoot at, as well as slowing down waves.
The specialty buildings are permanent.
Keep in mind they all have a disadvantage too, they improve something and give a negative bonus to something else, so buy only those who fit your strategy!
As an extra limit, you can only have one of them active at any time, so if you buy a rifle range and a super monkey lair you can only have one active, and not both. You can change wich one in the menu. Changing specialty building mid-game is not possible.
Best Answer
http://bloons.wikia.com/wiki/Monkey_Sub
Placed in water, it throws darts on bloons at medium speed(1.32 times/sec). It can be upgraded to have nearly unlimited range (by being able to target all bloons within the radius of any other tower), detect camo bloons, and to launch missiles (with an ability able to destroy up to a ZOMG in a single blow).
This tower has one of the best power/price ratio, and is the only one able to one-shot a ZOMG.