I like tower defense games, but it's a crowded market. You'll probably need better graphics or really amazing gameplay. One thing that's critical in my book:

I like tower defense games, but it's a crowded market. You'll probably need better graphics or really amazing gameplay. One thing that's critical in my book: "Normal" mode should beat new players regularly until they get the hang of the game. It's boring to win easily. I'd go Android and give the first x levels for free rather than in game purchases. Edit: typo