A unique idea made redundant..
By itself. It's actually quite a decent game. But it's just kind of redundant.
I'll echo most of the comments I've read - there's just too much money coming in. Everytime a new floor gets added there's immediately enough money to get another elevator going and defend it without ever needing to move anything.
The uniqueness of the entire concept is made redundant by the game itself. As interesting as the idea sounds, it doesn't actually happen.
What I would suggest is either make the elevators way more expensive or as suggest previously, add a cap to how many you can have. Or make it as such that you can only buy one less than the total number of floors.
And it makes things more fun. I actually tried playing the game without decking out every floor with a powerful turret and tried managing the movement of elevators around as well. It was actually quite interesting from a micro managing point of view. But way too hard to do so actually because of the speed of the entire ordeal and the seemingly unending flow of enemies.
To ball it all up into one suggestion, take the lift idea and make it in such a way that moving is compulsory (whichever way you choose) and design the levels in such a way that the moving becomes a more major part of the game. Less endless stream waves and more tactical movement. Have clumps of large enemies appear on certain floors at certain times instead of just one steady flow of enemies on every floor.
That's the only thing missing from this game is that the idea behind the game that catches attention pretty much goes away right from the get go. Find a way to make the elevator moving the center of the entire game concept and this will set it apart from the other defense games (which was the initial idea, I believe)