I hate tower defense games with a passion, but dear lord this game is amazing, I don't even consider it a tower defense. It's got an adventure mode, challenge mode, multiplayer, trading, daily and weekly mystery gifts, Pokemart, and a Pokedex, and it saves online so you can access your account from anywhere.
I could continue typing up this three page article, but you should just play the game for yourself, it's great.

http://www.playtowerdefensegames.com/games/1078/play.html
http://pokemontowerdefense.wikia.com/wiki/Pokemon_Tower_Defense_Wiki

And on a side not I've got a lvl 18 Mankey I'm willing to trade. It know Focus Energy, Fury Swipes, Seismic Toss, and Karate Chop.

Game's really old actually. I used to play it a while ago, and I occasionally play it, and it is still amazing. I've got all Pokemon with maxed out levels. Well, all that I use on my team.

Oh yeah. I played this when it came out. After my save files got deleted, though, I decided to move on to grinding Pokemon on my White Version instead. I agree though, great game, and only improving as time goes on. There are a few levels I needed walkthroughs to beat though.

I'm pissed they took down the passwords