Leveling in LioDen is based off of your lion's level. In order to level up, you need to gain experience (commonly referred to as 'exp'). Experience needed to level is based on a curve shape as shown above. There is no level cap in LioDen. This graph refers to both your lion and lionesses.
Lionesses and Lions both will need 1000 exp to hit the first level up. Then the curve kicks in, making it easier at beginning and getting a bit harder as game progresses.
How to gain experience