With the exception of Defense Grid: The Awakening, I've yet to find another tower defense game quite as enjoyable as this one. Revenge of the Titans has a lengthy campaign in which you unlock new technologies after each mission you complete and the currency you gain from each mission carries over to the next so that if you do well enough (through eliminating the aliens and mining resources) you'll have more resources for the next mission than you would have had if you did poorly. Later on in the missions the difficulty ramps up a bit and different types of enemies start to appear which act differently or have stronger abilities than the basic aliens which adds a decent amount of variety to the situations you will encounter. Revenge of the Titans also has a survival mode and an endless mode which adds to the overall replay value of the game. If your a fan of tower defense games then i would strongly suggest taking a look at this game, it was well worth the price and i got hours of enjoyment from it.