Unfortunately, there's not much depth here. Towers have to be placed on specific tower-spots, and on most levels, you're going to end up filling all of them (and, later on, upgrading all of them), so there's few actual decisions. You get upgrade points to upgrade stuff between levels but these can be ground endlessly and you'll quickly max out everything you use. There isn't much of a difference between the various enemies or towers; the game has some of the "classic" types of rocket-with-AOE, continuous laser, etc, but not enough depth in enemies or layouts to make this matter. And the levels are largely same-y from start to finish.