Hurrah for Hard Games!
As a wise reviewer once said, a game cannot be too hard, it can merely be unfair. Tower of Heaven is always scrupulously fair - it may be decidedly tricky, but every single death you suffer is always, unequivocally, your fault. Skill and perseverance are the only way to proceed, and personally, I love it to bits. What's the point in games being so unthreateningly easy that you can complete them on your first try without even looking at the screen?
Aside from the loving care that's gone into the design of the levels and the programming generally (you can control your character to a tremendous degree of precision, which is vitally important in a game like this), the whole thing looks and sounds smashing. The Game Boy graphics have a character and charm all their own, and the music is joyous, like something that the mighty Chris Hülsbeck might have written on a particularly sunny day. As for the ending, it wouldn't be out of place in a Hayao Miyazaki movie. Utterly lovely, and I shall be returning to see if I can root out the secret bits.