Leveling a kingdom can be done via the following:
- From the world map, select the kingdom to level up
- Press the 'Kingdom' button
- (The Kingdom Level tab is the default)
- Press the 'Level Up' button and pay the gold cost
Each level increases that kingdom's tribute by 1% and adds +1 to a Gem Mastery. Each kingdom can be raised to a maximum of Level 10, which also gets a bonus to life, armor, attack, or magic on all troops.