Besides cards, there are items that let you modify stats or gain permanent effects. You can carry out their recipes along with the rest of your loot. We're continuing to work on synergies and card "builds" to make them more interesting and effective. We wanted to avoid "leveling up" so as not to turn the game into an experience grind where you need +2 to damage just to beat enemies that were given +20 to HP. That's why each "world" is built on its own rules but within a shared balance that allows you to beat it even with a completely empty deck. If you're still finding it difficult, you can always lower the difficulty level. We don't restrict progression because of it in any way.