Oh yeah, for sure. All of the cards in the game and in your game are "tower cards". It should show a red X when you are NOT able to drop a card on a Tower because that Tower's slots are full. (Actually I just tried it and it does show a red cancel icon, but it is so small that it is not noticeable lol)
I do understand that it could be more descriptive in showing or explaining why you can't though, or even better, allowing you drop a card on an already slotted card to replace the slotted card with the dropped card.
Great idea on using some hover effects though!