Nice, but not excellent
A good defense game but need a few improvements.
6 differents tower without upgrades seems a bit light. The game need more interactivity than moving a really expensive tower all around the map (excellent idea that airlift). Music could be good also.
And I noticed a small bug, the game returned to the title screen and in the same time I still can move and buy units (same problem than Backspin), quite annoying when you play for a long time and.....................
It's almost a very good game. Keep the idea, improve some details and it will be perfect.