As you probably guessed, the more wealthy you become - the more you progress.
Your level is based on the value of all your properties. (displayed on the Dashboard)
However, you don't have to worry.
Even if you sell all properties, your level will remain the same (however total empire value will drop).
Tip: Trending properties can be useful help in quicker advancing.
Tip: You receive 5 coins as you level up.