My suggestion is don't waste your time on anything but standard monkeys. Start out by placing them in between the two "lanes" right in the middle of the map. Eventually, you should work your way in expanding the number of monkeys surrounding your original two.
I completed the game with 53 monkeys all piled around first half of the map.
The game becomes a little monotonous after two or three minutes. It would be interesting if the orbs started moving, or there were some sort of power ups.
Still, good for a few minutes as it is quite simple.
 "Bloons Tower Defense"){kind=icon}
 "Yak on a hill: Yak Gunner"){kind=icon}
 "B29 Assault"){kind=icon}
 "Biggification"){kind=icon}