Of all the deckbuilding games on itch.io, this is definitely my favorite. The cost/power ratio of the cards is very well thought out, and I enjoy the strategy between building a low vs high cost deck. I hope you make this into a mobile game!
Some random thoughts/suggestions:
Once I have bought all the cards, I have a bunch of coins saved up with nothing to do. Perhaps have the character skins (in profile) cost coins, or have an unlockable recoloring of the background.
Achievements would be nice to have in the future. Not just simply "kill x amount of a certain monster" but have achievements provide challenge such as "reach 20 attack in a game" or "complete a run without losing health" or "spend x energy in one run" or "kill every monster (with health) in one run using weapons" or "only use cards costing 5 energy or less in one run" or "win 5 runs in a row". These achievements should inspire different deck builds and adds replayability.
Maybe have a gamemode with 2x the deck size or somehow incorporate different "levels" that you progress through.
Speaking of replayability, it may be possible to add different player classes that would either start with different stats or have different restrictions. (Ex. a class that cannot gain more than 3/5 health but all weapon cards have double effects)
Have a "beastiary" button that shows a collection of the monsters. This would help when deck building and understanding some of the card effects.The bard card, for example, references the rogue monster, but players have no idea what the rogue monster does until they encounter one in a game. There are other cards too that reference monsters/hazards in their description that could be reworded.
A few of the card wordings are unintuitive. Last stand says that it plays from the deck, but it will also trigger from the discard pile (a good thing). Would also be nice to have a "status" bar to see which react cards are waiting to be triggered so I don't have to keep a mental note that a bard/last stand is in my discard pile.
Robes of faith should be reworded. "Prayers will always be answered" makes sense once you understand the game, but maybe reword or change mechanic so that prayer cards always roll the highest amount. This will make the card worth it (currently one of the weaker cards).
Bandit king wording should be reworded.
Hardcore game mode should have a description (are there more hazard cards or are enemies stronger?)
What is the "A" in the corner of some cards? Perhaps it is obvious and I don't realize; I have been playing perfectly fine without knowing.
The pass button colors are confusing at first. The grey and blue colors should be switched so that blue means you can trigger pass and grey means it is "greyed out" and unable to be used.
Cards should have a different color depending on their class. (Ex. all react cards, all alchemy cards, etc).
Cards that could be buffed:
Bard:not worth taking up a constructed deck slot, should change to normal card that costs energy that removes all rogues.
Death: could also add attack when triggered.
Ring of reflexes: not enough react cards to make it interesting
Bandit king: cost should be lower, since you are only getting 6-12 attack from killing the bandits after. Weak compared to other high cost weapons.
Life: Does not work after using death (ironically), meaning the +10 health doesn't add.
Ring of curses / Staff of curses: seems like cost should be lower, but I haven't used them very much to say if they are balanced.
Skeleton key: cost could be lower. Again, I haven't used this one much so it could be overpowered in certain builds.
My favorite mechanics/cards:
Lucky clover: Building up a high energy base and using this card 6+ times in a game is very fun.
I enjoy the RNG of the rewards with Armory, Summoning, and Ring of Wealth/Fortune.
Staff of charming: very cool concept
Sword of sacrifice: fun to decide whether it is worth it to make the sacrifice or not. I would love more sacrifice cards in the game.
Boots of silence/Robes of stealth: strong cards and fun to use.
My current build. Goal is to build up energy quickly to trigger the strong cards (armory, lucky clover+reward). Once these have been used, trigger "Life" as the win condition near the end of the deck to tank any remaining monsters if I have no attack. This build may need another early game card.
Mushroom, Loaf of bread (health to stay alive before getting armory)
Energy tome x2 (to build up energy quickly)
Boots of silence x2 (to remove curses, poison, rogues)
Lucky clover: fun and powerful when having a high energy pool to use the rewards
Armory: enough attack cards for the rest of the deck
Life: enough life to survive the rest of the deck
Like I said, this game is incredible, thanks for making it.