Revenge of the Titans is described as a Tower Defense with RTS elements, tower defense on wikipedia is described as an "RTS offshoot" (which is also how I classify tower defense) so for me, the way they describe the game was pretty strange until I started playing it.
What sets this tower defense apart from all others is two major points. The first point being that like in a normal RTS game you must gather resources. You do this by placing a gathering building near the crystals to mine them. The second point being there is a research system. Normal tower defense, once you get past the first level you unlock another tower, and possibly another after the next one and so on and so forth. In Revenge of the Titans, after each level you pick a technology or a building. Each building you can choose requires certain technologies researched, and some technologies give you bonuses to damage, armor penetration and such.
Mostly other than that it plays out like your standard tower defense, you spend your money (gained from pickups, killing enemies and mined resources) on buildings and your leftover resources (along with the level completion bonus) are carried over to the next level, so in this game, hoarding resources is actually a good thing as it will help you out later in harder levels.
I have only played about an hour of the game and have not seen all of the enemies yet, but there are armored enemies, enemies that seem to move by leaping then pausing, leaping then pausing, there are larger, stronger versions of enemies and there are boss enemies. There are probably other types of enemies in the game that I haven't come across yet (possibly flying enemies?).
All in all I have been really enjoying this game and would highly recommend it to any Tower Defense fan or someone looking to get into the genre.