Even though the tower defense genre has grown stale over the years, some developers understand that there's still some life in the genre if you play around with the formula. On the one hand, you have a game like Sanctum that changes the perspective to first-person and adds the player as a weapon. On the other hand you have Anomaly: Warzone Earth, which turns the genre on its head to give you a reverse tower defence strategy game
Instead of building towers to force units through a path, you create a path for units to destroy towers. And by doing so, 11 bit Studios has created one of the most enjoyable strategy games in recent history.