Nice game (except the boss monster which is totally unfair: completely unexpected, you can't backtrack and if you die you cannot reload). The REAL problem is that the game CRASHED twice! It crashed while saving (once autosaving, once when I explicitly saved). A command prompt appeared but I could not type anything. Sorry I forgot to take a screenshot. I reloaded the page in the browser (I'm using Google Chrome 80.0.3987.132 on Windows 10), I loaded the last saved game, it worked but I lost ALL my inventory! To add insult to injury, the first time I reloaded I was "magically" placed in the cave level OUTSIDE the walls, without tools. I restarted from scratch, I was very near to the endgame, I had all the benches, all the gem tools, and... CRASH, again, lost everything. I gave up :(
About the gameplay: I liked it, but the progression wood/stone/iron/gold/gem is too unnecessarily long and time-consuming. If I were you, I'd remove gold completely from the game and go straight from iron to gem, or maybe you could use gold for something else (after all, the idea that gold tools are better than iron tools is ludicrous EVEN for a game that has zombies and magic pockets).