What's the difference between the level of the player, the level of the Castle and the level of the population?

There are 3 types of levels in this game:
- the level of the player is stated in the green experience bar in the upper left corner of the game screen. You need only experience stars to raise it.
- the level of the Castle is stated in the menu of the Castle, on the right page ("Castle level"). You need to increase the amount of citizens, get resources and finish other tasks to raise it.
- the level of the Population is also stated in the menu of the Castle, on the right page, in front of the Population bar. You need only to increase the amount of citizens to raise this level. You can see the amount of the citizens you have and the necessary amount of citizens to upgrade to the next level on the Population bar. Each level of the Population gives you more coins in the Treasury.