There isn't anywhere in the tutorial that tells you how cards-per-deck limits work in this game, you should probably add that. Adding the ability to sell your cards that go over the limits would also be a good feature. Additionally, the deck-building interface needs a lot of work, but if I had to guess, that's probably because engine limitations makes it harder to change that particular aspect.
Edit: By the way, I like the game a lot so far. I find myself saying this a lot, but it does have great potential. The armor system and multi-phased battles seem quite unique, honestly. And the way things are organized makes room for a lot of unique combos and synergies that could really allow you to create a great variety of cards.