You will start in this Empire. This Empire has 21 buildings which will unlock when you progress in the game.
When you reach level 100 you will unlock Empire II. This Empire has 22 new buildings which also will unlock when you progress in the game. You can fly between the 2 Empires using the airport.
When you reach level 175 you will unlock Empire III. This Empire now only has 4 buildings but new buildings will be added in future updates. You can travel between Empire II and III using the secret cave.