The whole foil card concept is a little bit on ice right now :-/
Internally I'm working out a few concepts already how foil cards should be gathered. But this will be a suprise in the end ;-)
Depending on how they are gathered it could be cumulative or not...
But the original idea was that each player profile has attached wich foil cards this profile owns. Not the amount of times the foil card was won.
So it would be like having a permanent power up? Then I'm not liking this much. :/
I understood that it was a 'level 2' tower. I wanna share a new concept for this.
-Everytime a tower makes 50 levels on its own (no replacing) , it becomes a foiled card. (+ 1 foiled card ladder, + X exp).
-As a foiled card, the tower gains a bonus. (what @Thomadin suggested).
+ Extra: A section in the menu where one can see how many and what foiled cards he/she obtained. The appearance would be like the normal in-game deck.