The Level of a card (seen in the upper left-hand corner of a card) is the amount of mana needed to use that card. In order to pay the cost, you Tap cards in your mana zone until the amount reaches the number on the card.
You must have at least 1 of that cards civilization in your mana zone to use the card, however it does not need to be tapped to pay for that card.
The Mana Zone (placed as said above) is where cards are placed upside-down to pay for a cards level. There is no restriction as to what civilizations can be in your mana zone. On each turn, it is optional to place a card here. If you do, you may only place 1 card here at a time unless dictated by a card effect.
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