Like the idea, could be refined
I was enjoying this until I reached the witch. I had no money after just buying the Explosive upgrade, and all my potions ran out so I couldn't attack. The witch was not fighting back, so nothing was happening and I was forced to restart. Not good..
I think the potion mixing concept itself was a bit flawed too. It was very fun at first to try out all the different potions, but when I found one that worked well I just stuck with it and had no need to change. Would have been much better if there had been a need to change (which it looked like you'd done with the witch, had it worked for me).
Graphically it's good on the whole with a few areas for improvement. The canon reloader for example, circular gradients like that looks horrible, especially as it contrasts with the rest of the graphics.
I'm a bit sick of defense games now, but the original concept of this one gives it enough interest. I think you could have taken the good idea further though. A nice effort, like your work a lot, keep them coming.