A card can be evolved after it reaches level 20. You will also need a large amount of gold coins to evolve the base monster, and more for rarer cards. Evolution also has a required material, which varies by monster. Usually in order to evolve a monster, you will need an identical card of any level. However, some cards require two or more other cards to be evolve. The rarer the card, the more cards that will be required. To view what is required to evolve, go to the "Evolve" screen, then place the monster card in the Base Monster slot. Evolved cards have the same skills as their original version, but have better HP and Attack stats. They also can be fused further. Evolving your leader first is recommended, as you will receive its identical card for evolution when you reach level 20. After that, evolve the monsters that are most useful, even if they are of Common rarity.