The towers plays the best role on Bloon TD (Tower Defense) strategy game. Each tower has its own difference so you can decide your own strategy and making your favorite combinations. On the first sequel of Bloons TD, there were only five towers available like:
  • Dart Monkey
  • Track Shooter
  • Super Monkey
  • Ice Tower
  • Bomb Tower
As the installments of new sequels of Bloons Tower Defense, there are new towers introduced and has its own uniqueness. On the new Bloons TD 5 there are two paths to choose from starting at tier 3 upgrade which has its own use depending on the stage situation. If you are unsure on which path you will choose, always refer at the help button (question mark icon).

Here are the list of available towers on Bloons TD (Tower Defense):

Tower Picture Description
Dart Monkey

Shoots a single dart that pops a single bloon. Good for early rounds and cheap (see upgrades). 
Tack Shooter

Shoots 8 tacks spread in all directions which can pop 1 bloon. It has a short range and medium-slow fire rate (see upgrades).

Sniper Monkey

Armed with a high-tech long range rifle, pops 2 layers off of bloons with unlimited range (see upgrades).
Boomerang Thrower

Throws a single boomerang in an arc back round to the monkey. Each boomerang can pop 3 bloons (see upgrades).
Ninja Monkey

Stealthy tower that can see camo bloons and throws sharp shurikens rapidly (see upgrades).
Bomb Tower

Shoots a single bomb that explodes in a radius burst on impact. Good range, medium-slow fire rate and can pop lead bloons but not black bloons (see upgrades).
Ice Tower

Freezes bloons in its bursts in its radius for a short time. Frozen bloons are immune to sharp objects (see upgrades).
Glue Gunner

Shoots a glob of monkey glue at a single bloon. Glued bloons move more slowly than normal.

Monkey Buccaneer

This tower can only be placed on water surface. Shoots a single, heavy dart that can pop up to 5 bloons each.
Monkey Ace

Patrols the skies above the action, regularly strafing the area with powerful darts in 8 directions.
Super Monkey

Throws darts incredibly fast. Has long range and lots of insanely powerful upgrades.
Monkey Apprentice

Trained in the arts of monkey magic, the Monkey Apprentice weaves magical bolts of power that pop bloons. Each shot can pop 2 bloons.
Monkey Village

Does not attck bloons but lowers cost of all towers and upgrades in radius by 10%. Has many useful upgrades that help nearby towers.
Banana Farm

Grows bananas that you can collect to turn into cash. When your farm produce some bananas, collect them by moving your mouse/tapping the screen over them. Don't leave them too long cause they will spoil.
Mortar Tower

Targets a specific bit of ground anywhere on the screen. Launches explosive mortar shells to that spot. Useful for placing far away from the track to make room for other towers.
Dartling Gun

Shoots darts like a machine gun, super fast but not accurate. It will shoot whatever your pointer is, so be sure to control it properly.
Spike Factory

Generate piles of road spikes on bits of nearby track. Each pile can pop 5 bloons, and unused spikes disappear at the end of the round.
Monkey Engineer

Good at building stuff, the Engineer shoots bloons with a powerful nail gun and has many useful upgrades that create traps, sentries and other enhancements.

Rapidly sucks up and shreds bloons, spitting out what's left out the back.
Heli Pilot

Powerful attack helicopter armed with twin dart shooters and multiple patrol options.

(Credits to Bloons Wiki)