Ya it does start to get crazy at the end, by the last level I think I usually have 4 or 5 of the Super Monkeys. I'm trying to beat it without losing a life but I haven't been able to yet...
This game spawned from custom wc3 games called tower defense, of which there are hundreds of varieties and have been around for years now. If you really enjoy that I'd suggest loading up your copy of wc3 if you've got it and looking for tower defense games in the custom game menu. Though as of late most custom games have been completely been over run by DOTA.