I think that a tower defense game needs action and lots and lots of strategy. Take plants vs zombies for instance or last stand. Those games have been played for generations. Don't make your game a stand and place game though, because people have been making those types of games for years. Try make it to where you can walk around, make it to where you can also defend whatever it is that your defending. People want to be apart of the action, people want to be constantly in the game, not just standing around and watching. People want the constant panic of defeat being imminent if they don't do something. I hope that helps.