So, finished playing and here's my review:
1. The art is really done well, although I would love to have better animations for actions. But the variety of enemies is complementing the lack of animations for me. Music is amazing, a lot of biomes is also a nice touch, in general this feels like a lot of content (I only counted 3 biomes and 5 enemies, unsure if there were more)
2. The difficulty of the game is nicely balanced, I had time and opportunities to heal and sustain damage while waiting for damaging type of card.
3. I haven't really noticed visual indicators. I believe that's because of custom engine, but would be nice to have at some damage numbers or heal numbers appear above enemy/player. It was also difficult to know what each type of card does - I'd say its better to have at least a text above them to say that some of them are damage cards, and others are healing cards.
4. I have noticed some bugs, probably it's also related to the custom engine. For example, sometimes the card at hand is hidden and I had to manually drag them to know what cards do I have
5. At first I didn't really understand the mechanics, but it's not very difficult to manage without a tutorial.
Overall, it's very impressive considering the custom engine you used. Keep making games, I want to see more of those!