Great game, but there are issues.
Of course a rogue-like is supposed to be challenging, and you've done a great job of doing that. But the challenge should come from the gameplay, not bad mechanics and balance issues. I develop table-top gaming systems, so I have a few suggests for how to make your great game an amazing game.
Better balance. I'm not the first to comment and probably won't be the last, but the rogue and warrior classes are basically useless compared to the mages with their unlimited ranged attacks. Perhaps give them a crafting skill tied to the Ranged ability. Spend energy to craft darts for the rogue and javelins for the warrior. Rogue could make like five arrows and warrior makes maybe two or three javelins. Since we're spending energy all the time, I think that's a fair trade off to give the non-mages more access to ranged attacks.
Food. No matter how powerful we get our characters, once the food runs out the game is essentially over. Since there's no reliable way to obtain more food and the merchant doesn't restock his wares, there should be some way to obtain food. Maybe a "Hunting" skill that you can activate like the Sneaking skill, and while it's active it greatly increases the chances of monsters dropping meat.
There needs to be a sell option with the merchant. I like how when you pick up more than one of the same item, it gives that item a +1, but ultimately you pretty much stick with one or two weapons. Yes, they all have different uses, but we should have the choice of whether or not we want to make use of those options. Plenty of times I just wish I had been able to buy more meat but instead I'm carrying enough boots, gloves, and spears to stock the clearance rack at at Renaissance Faire. Also, the merchant should sell keys.
Rings. The weapons stack and improve on each other when you pick up multiple of the same type, so how come rings don't do that? It would be perfectly fine to NOT make the rings stack if we could equip more than one on each hand. Last I checked, I have five fingers on each hand and I could fit a ring on all of them if I really wanted to.
Those are my ideas. I really enjoyed your game but ultimately it became more frustrating than challenging due to the various issues I mentioned above. If you are able to address those issues and make improvements, I'd gladly come back and waste an entire weekend wearing out my left mouse button.