My finger hurts
First off, definitley a unique take on the TD concept. I enjoyed the whole shaft protecting thing. The monsters were okay but there were to few, which leads me to my next complain: Repetitive. The game was extremely repetetive, with more monsters and more elevators being the only things I could look forward to. Forcing players to be strategic in the placements of the lifts while acting quickly made for a nice idea. It was nearly frustratingly hard on my finger as the key to survival was your own gun. Funny enough, I told myself I'd quit after the 15th level and was pleasantly surprised when I saw it was the last level, so maybe add in your comment that there are 15 levels, just incase someone like me decides to give up one level to soon. Some weapons (for the player himself) would add to the awesomeness of the game. Lastly, always remember that a disturbingly expensive, but all-powerful weapon is a must for all TD games (Which would work especially well in this game if limited to one)