Boring, repetitive, and most of all, easy.
Now, I feel like a bit of an ass here. Everyone seems to be giving this game a decent score. Nothing wrong with that at all. It is personal preference after all. For me though, when I play a game... it has to be enjoyable in all aspects.
For example; I found Primal Defence to be absurdly boring and repetitive. It is the exact same thing every level. You are on the left, the enemies come from the right. You shoot your arrows. The arrows hit your enemies in the face. Enemy falls down. Every so often you have to shoot something out of the air.
You have a good idea here, although you need something to add life to it, make it less repetitive. Add a boss monster at the end of a wave of enemies. Don't have an infinte amount of specialty arrows/bombs, and let them do elemental type damage. For example, the fire arrows light your foe on fire, as well as any enemy touching him. Ice arrows slow your foe down. Elemental damage, combined with a limited supply of your specialty arrows, would add an element of strategy to this game. Add an increasing (and noticeable) difficulty level as you play through.
I do like the graphics though, as I think they are well done. The enemies don't look the same, the backgrounds are very nice, and it did not look like a cluster**** of random crap.
Now, I must give you a 2/10. Again, I am not trying to be an ass. I think you have a good idea that can be improved, and built on. Good luck on anything you work on in the future!