Fun, but flawed.
This game is all about the gameplay, as with most tower defence games there is little emphasis put on graphics and no storyline in sight. Unfortunately, there isn't very much gameplay to speak of here. Sure, it can be fun clicking repeatedly to mow down rows of enemies, but that's all you'll spend the game doing, as the entrenched weapons aren't up to much more than being a stopgap.
Which would be fine, except the lag in the game kills this principle stone dead. Particularly with the addition of the fourth floor, the lag in the game makes it almost impossible to successfully move elevators between floors; basically, most games will finish with you watching a floor become overrun while you curse and frantically try to click and drag elevators around when the game thinks you're trying to shoot (perhaps mapping the "shoot" and "move" commands to different keys would help with this).
Fun for a few minutes gaming, but not something you'll play repeatedly.