Great game. Really enjoyed myself until castle 3. Afterwards it became more level and rep-grinding and thus boring. I did completed it tho. However, this game needs some improvement. Good luck with that.
Also, you can use your bubbles for defending your own bigger ones. Oh and did I mention you can use everything at once by just double-clicking? Everyone who says that this is hard, isn't paying attention to the tutorial or are doing 3 things at the same time. Just follow what "beatsstayflowin" said
I played until lvl 13 with ease. Level 14 and 15 are a bit tougher. Not so hard game in my opinion.
As long you keep the enemy under control (and the extremely large bubbles, don't go for them at start btw) then your fine. Nice game.