Thank you for the input and the heads up!
On the feature list there is Foil Cards, which are very close to achievments!
For every tower in the game you can win the foil version of it! If you own this foil version the tower card will be golden when found in the game (like foil cards in CT games)
In the main menu you would have the option to see all your unlocked foil cards.. Plus greyed out the cards you still need to unlock together with the objectives to do so.
would having a "foil" of the card do anything besides showup ingame different color when found? like small bonus gain per lvl for instance