I got into Tower Defense games by playing White House Defense on Kongregate. THen I found Bloons TD III on the same site and I played like every level at ever difficulty.
It was really tough on my old computer - I had to ask my Dad for a new comp so that I could play the final level without them freezing up completely.
Then I found Gemcraft and I've never played Bloons again....
Now I hardly ever go to Kongregate anymore. besides I like the forums here better than I like the chat over there. Often people fight and say bad things in chat and it's right next to the games you play...
BTD3 is very fun, but it's also quite challenging. Very creative in my opinion. Wish there were more choices for towers and more money each kill though.
The BTD series are always fun games to play and are normally my first pick when I have nothing to do. BTD3 is definitely my favorite. I can't wait to see more.