I have always wanted to make a tower defense game, interesting to combine it with a card system! I was not a huge fan of towers being destroyed with how the game currently is, price is so expensive for them that I felt as soon as it did die if I placed another I found it often died immediately and it was a waste. Having upgrades with such a short round length is also a bit tough because before you can really buy or have any sense of them the round is pretty much over. Cool what you are aiming for and I am sure you can continue to make interesting combinations of games!