Nice concept
The only thing that really bothers me are the controls. The highlighting feature is cool but it gets annoying when i want to move my units but accidentally dragging a little causes them to deselect and then I end up losing the game. Make it so that the player holds ctrl and drag to select or a certain button so this won't happen.
Other minor issues were from the upgrading system. i found it annoying how small the buttons were and the fact that you don't see the costs of the upgrade until you click on it.
You might want to add a volume control for the sound fx and music too.
Its a nice game with a couple of flaws.