I've got to say I don't like tower defence. Strangely You made it that You took control from the player to... give him more control? This arcade version is very awesome, I like it:D
Though, I have to say the game is pretty easy, and it basically plays itself.... I know there was not enough time, though I think it would be better if player could have different cooldowned weapons, like bombs, rockets, etc. Or make something stranger like aliens having armor. Tower can't destroy armor - but You can. That way both of those gameplays matter.
My play was shooting everything in the beginning, lazly shooting everything later, and in the end only collecting coins, towers destroyed everything. But I guess its just balance thing...