Complete level 4 to unlock the Forge. The Forge allows you to combine cards to make one of them more powerful, as well as evolve cards. When you evolve a card, it will gain a new upgrade option during a level. You can spend money to upgrade the tower during the level in order to make it more effective. To improve a card, you will have to sacrifice an unwanted card. To evolve. you must have two of the same card.