Play Cooperative with friends, trying to hold back the hordes as long as possible while getting a high score.
Try Efficiency mode if you're a tower defense pro - kill the enemies as soon as the come on screen for a higher score.
But if you're truly a badass, try out Gambit mode. In this mode, you get more gold when you kill the enemies as close to the exit as possible - but if you mess up, the consequences are game-ending!