Fun idea. I like tower defense games, so it was fun to play with this and watch my pieces defend or attack.
I'm not sure why, but often my drag+drop attempts didn't seem to take affect. (For example, when I drag an attacker to an unclaimed attack position, there's a ~60% chance the game acts like I didn't attempt to place the card at all.) It looked like I could afford to place the card, so it felt moreso like an issue with the drag+drop UI logic than a feedback issue where the game isn't telling me why the play is illegal. But whatever the case, if that were fixed up, I could see myself playing longer and enjoying it.