It's obvious that you put a lot of effort into this game, and I applaud your creation. However, the flying creatures ruin the experience, especially since they can simply drain the player's life all the way down if you happen to stay under them. Shame.
The difficulty curve could be ramped up somewhat. At the end I had almost all survivor slots filled. Maybe add bosses? Very similar to Last Stand, I guess it's an homage.
If you click on the charge bar for any unit it will bring up a popup with all the modifiers on them