A great game..
I liked the entire premise of the game, controlling the archers to attack the oncoming relentless swarm of unholy goblins. The upgrades were great as well, just hold down a key and it spams the upgrade. Also the healing, so instead of repeatedly clicking the same spot you can let the mouse hover. The power-ups were great as well, whenever you got them you had the power to take down anything that comes in the general area.
I enjoyed the game a lot except for a few minor touch-ups here and there. Such as the 'Enter' key reverting you back to the main menu. Where you then realize you have to start all over again, when no mention of what the enter key does was ever said. Something more practical such as the Escape Key, since that has no use.
I also thought you could use a 'Predetermined Flight Path' for the easy level, the idea is to have a faint line go out from the archers to the area on the ground that the arrow fired will land. Only because I had a large amount of trouble getting used to it and anything that can get me to visualize where everything is going to go can only help.
But I hope to see another Goblin Defense because I thought what you had done with it so far was great. Keep up the good work.