I liked this quite a lot. The implementation of the theme was really cool. I liked the idea of jumping down into an abyss. Having gold reset after jumping back into the abyss was also a nice way of making sure players are incentivized to upgrade uniformly if that was the intention. Level generation was good. Sound was pretty well executed too.
However a couple of issues hold it back real bad.
Having attacks and mining share the same resource is cool but with no reliable way of recovering energy, it was quite frustrating to be killed because you couldn't do anything against the two birds tracking you down in the hallway. I feel the birds had too much tracking too but that wouldn't be a problem if the attacks issue is solved.
Additionally, having some invincibility frames whenever you take damage, being able to change where you aim while mid air, removing diagonal tiles because of how you can't attack enemies reliably on them and increasing the height of the corridors by a tile(to make jumping over enemies easier) would've helped the game by leaps. I know these are fundamental elements but they were sorely missed in my experience.
I hope this doesn't come off as too critical. I liked the flow of the game enough to spend more than an hour on it but the smaller flaws just held it back so much.