I had some trouble with the game lagging and the scaling interface, so my gaming experience was not optimal, although I think that the concept beneath is very interesting; I talk about crafting a creature and then controlling it to fight. Good premise, original.
For the menu, I would place a clear PLAY/START button, as it took me a while to realise that the screen was the target I should click to play.
Is the player meant to have any power from the beginning? I was able to just dash and hit with my sword. It might be useful to place a small tutorial and give the creature some basic powers, so the player starts getting acquainted to use keys 1, 2, 3 and 4. Would do the thing with the creation interface; although he shouldn't be able to pick the most powerful limbs from the start, it would be great to be able to chose in between slightly stronger or faster arms, for example.
It is a personal thing that I hardly stand whistling, so beware that could drive some players mad xD It happened in my proposal, my techno music gets to repetitive, so I added an M key to switch music ON/OFF.