The equation for experience gained is calculated as follows = (EnemyExpReward * (1.0 + NewGameLevel * 0.4) + 30 * NewGameLevel - PlayerLevel * 3) * ExpMultiplier
EnemyExpReward = base exp rewarded by killing the monster, for specific values see the monster wiki pages
NewGameLevel = the current difficulty setting of the run in progress
PlayerLevel = current level of the character you are playing
ExpMultiplier = %experience gain enhancers that come from items and buffs (all stack additively)
The level cap is 20. You no longer gain exp once at level 20. The level cap increases by 5 for every time the character beats the game and goes into New Game plus (aka NG+).
Example: NG+ = 25 level cap, NG++ = 30 level cap
You gain 3 skill stars each level up that you can spend at the guild hall to upgrade your skills. The number of starts gained also scales with every 5 levels (see example below).
Example: Leveling in levels 2-5 = 3 stars every level, leveling in levels 6-10 -> 4 stars every level, leveling in levels 11-15 -> 5 stars every level etc. (continuing on with the same pattern)