There are only two I like. Gemcraft and Bloons Tower Defense 3. You see, people have a tendency to make TD games EXTREMELY HARD.
Very true, but hard TD games are fun. It creates a good challenge, and makes you kill a lot of time (In a good way). They're great time wasters.
@ Everyone else: Any other tower defense games you can think of that aren't on the list, here? If the game is very bad, though, I may not want to put it on the list. But most TD games are good.
