Cool game. I like the added difficulty level of the caves. I got zero collectables at the end though so I must have missed those!
I didn't get the fact that it was lava until I read the comments but that didn't spoil the fun. Not knowing what it was made it even more interesting. This part really made me think about when to kill those enemies :-) The only drawback is that 'learning' this is quite expensive since you lose life. Maybe you can play the hurt effect the first time without actually taking life.
Overall quite varied and I had great fun playing it until the end!