If you have, then tap cards in your mana zone equal to that creature’s level and put the creature into the battle zone. The level for a creature is in the upper left corner of the card. You pay a creature's level by tapping cards in your Mana Zone for mana. If you are unable to pay for the level of it, you can't summon the creature.
There's no limit to the number of creatures you can have in the battle zone. You can only summon creatures on your turn, and only if none of your creatures have attacked yet. Creatures can't attack on the turn you summon them into the battle zone (unless an Ability such as "Fury Charge" says to do so).
How to Play