Thank you for the feedback. Game balance with regards to costs, tower damage, and enemy health is definitely something that I need to put more thought into.
Regarding the cards not locking, they do lock for that particular draw, but the game then clears the locks and sorts the cards for evaluating, I did it this way because I did not feel like figuring out how to make the locks move along with the cards and the evaluating needs the cards to be sorted, but as most players think that the locking does not work, I will rethink how the game handles this.