Gameplay: Requires and rewards strategy and problem solving. Two game modes really helps break it up, and gives the game much more depth. Ability to combine any tower with any upgrade is freeing. Each enemy is unique, requiring you to think on how to best counter each one.
Graphics: The graphics are VERY pretty eye candy and fit the theme. Love the enemies' unique looks/animations. I also love how the towers aim at the enemies and animate as well. The NPCs look amazing, and really bring you into the story (especially the easter egg NPCs).
Sound: Love the high quality sounds/music. It really fits and feels right and I love the beat. The laser sounds are average, but how can you improve that?
Story: The fact that a tower defense game has story is amazing let a alone a good one. It keeps you wanting to see and play more.
Overall: Perfect sequel! You took the first game improved it and polished it to perfection. This game easily deserves the highest of scores. Dare I say more please?