Enhancement is the process of using Unlike cards and Dorri to increase the level (and power) of your Base Card. All cards other than the Base Card are destroyed in this process. Using rare or enhanced cards to level up your Base Card does not make it more powerful, but will grant more card experience than using common cards.
During the enhancement process you can choose to feed up to 10 cards to your Base Card. Some cards have Skills and if some of your feeder cards have the same skill, the Base Card's skill may increase, up to once per feeder card with a similar skill up to a maximum skill level of 10 (will display "Lv MAX" instead of "Lv 10").
NOTE: Event specific skills do not level up by being fed similar skilled cards, they instead start at level 1 for a base card and gain a skill level for each new image art gained (subsequent evolution). Skill level 4 is gained at final form evolution and skill 5 is attained at final form maxed enhancement.
The cost (in terms of similar rarity cards) to level up a card increases exponentially, that is, whereas enhancing a level 1 Rare card with 10 common cards may increase the level to 9, using those same 10 cards when the card level is level 35, might increase it only 1 level.
The card statistics do not necessarily increase the same amount for each level, that is, if you increased your card level 6 times and then do it again, the statistics increase the second time may be different than the statistics increase of the first time.
The Dorri cost per feeder card during enhancement is determined by the current level and the rarity of the base card. As the Base card's level increases, so does the cost. The cost per feeder card is the same when fed all at once. It is better to do 10 card enhancement at a time rather than 1 card enhancements since this will save you time and Dorri.
When enchancing there is a chance to perform a Great or Excellent enhancement. These confer a 20% to 50% bonus to the exp gain from enhancement. From time to time there are short "EXP UP!" events for a couple days where all enhancements gain at least a 'Great' enhancement bonus.
The Enhancement information of a card can be found as seen inside the red rectangle below.
A cards Maximum Enhancement level is determined by its rarity.