especially if they have a high cost. As you advance through the different levels you'll find other, more powerful cards that let you inflict way more damage. You need to know how and when to play them, though, and be smart about creating a deck that lets you both attack and defend effectively.
At the bottom of the screen, you have a ball of energy that fills to a certain maximum. If you want to use more energy, you have to touch the ball when it's blue. You'll use the energy to increase the deposit, so you have to manage your strategy to put soldiers down and use the most costly ones when the time comes. The game is won when you reduce your opponent's tower to zero, whereupon you can advance to the next, more difficult level.
Enjoy playing this tower defense game with your deck of cards and destroy hundreds of enemies while evolving your soldiers to gain a slight advantage over your opponents.