There is experience (XP) in “Burnt Islands” that you gain by killing monsters and taking islands (expanding your territory). You can use it to get either more health, strength or speed with which you can fly. You are going to need more strength if you will fly long enough without falling down. The speed will help you fly faster so that you can avoid some of the enemies without getting into a battle.
Here is a screenshot of an experience menu that shows up if you press Tab. The first number is your current level and the second is a cost to upgrade to the next level.