If you tap a building you will see a pop-up window showing its current level. Under the level icon you can see a number of times you should receive an income from this building to upgrade it to the next level. When the necessary number of deals is completed you will see a star symbol above the construction. It indicates that the building is ready to be upgraded to the next level. Note, that you can always upgrade buildings immediately for a certain amount of coins or crystals.
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