Completed the first challenge on easy. You have to get through 100 waves of baddies..which is kind of tough considering I didn't really know how to play this particular tower defense game.
Basically, this game is all about creating a maze that your bad guys have to go through, stalling them as long as possible before they get to the end gate which takes lives away from you.
What I did was built up a TON of defenses near the entry gate, the gate the bad dudes come out of. Then, what I would do is loop the bad guys back and forth past those huge clumps of defenses, since it sort of gives you the most bang for your buck and you just have to concentrate on one particular area.
Overall, I like the game and I look forward to playing more of it. Sort of a shame that it took me so long to take it seriously...but at least now I can manually enter in game play entries at gamerDNA about it :P