This is an excellent piece of work, with great design and execution, worthy of your efforts. However, the rewards after each level are quite limited, making the game progress heavily dependent on luck, making it difficult to advance. I have played around 20 rounds and the highest level I've reached is 5, which is a bit depressing.
And though not sure if you're interested, my current ideas to improve the balance (also the whole gameplay) are:
- Offering 2 card rewards each round, this will help lengthen the deck-building process, which is the core of a rougelike game, and it will also improve the chance for players to get better cards (suggested by TheMixedMaster in Ludum Dare comment)
- Increasing mana recover from 3 to 4, this will allow players to play more cards each round, adding the chance for them to make decisions, this will also reduce the difficulty
- Increasing all units' health, this will give players more tolerance on bad luck, and will make each round longer to give players more chance to play permanent attributes modifying card