It's a little too easy I think ...
i managed to beat the game without losing a single hit point just using upgraded standard turrets and one flame thrower.
however, i like the design of the game and the idea of having more than one platform. and the resolution is better this time, isn't it? can't remember that well. i have to admit, i like the theme song too ^^ at the beginning i thought it wouldn't fit that well, but now i got used to it somehow. due to improved details the whole game-ensemble really makes a good impression.
well, i have to agree with some the other reviewers though, there really could be more strategy involved. maybe you should make some enemies, that are harder to defeat with standard turrets and can be defeated easier with other turrets since these upgraded standard turrets are the ultimate weapon in the game, you really don't need anything else.
btw, to avoid lag just don't build that many turrets and focus on upgrading.