Players may need to play a round or two to fully grasp how this card game works -- but the structure isn't overly complex, leading to quite a lot of fun. Essentially, players combine cards to either alter their numerical amount or remove them from the board in Cards of Terra, chipping away at the stacks beneath prized gold cards they need to free to win. Using logic, planning, and simple addition and subtraction skills, they'll try to figure out what combinations will work fastest, because they only have a certain number of moves. Further levels introduce new elements, such as the ability to use a turn to directly remove a card by dragging an arrow from the counter that lists your number of remaining moves -- or trying to navigate around cards that can only be placed within a limited radius.
The backstory, involving an alien princess using her powers to make enemies fight each other, is far clearer in the app description than when it's shown purely through images in the app -- but understanding it isn't really central to playing the game. The app has a notable focus on playing, and not things to buy -- players don't have to worry about pausing frequently for ads or spending real-world cash on app-related currency or other items. They can pay $2.99 to remove ads, although they probably don't need to, and there doesn't appear to be any other items they can buy. The biggest challenge involves freeing the gold cards before you run out of moves, which can sometimes be difficult. Once players run out, they're out of energy and will need to start from the beginning to try and beat the level. They can fail a seemingly unlimited amount of times, though, with no penalty -- and be able to try again -- which, coupled with the lack of purchase pressure, can make Cards of Terra a pretty pleasant way to spend some free time.