Two civilizations can enter the vassalization relationship through diplomacy (at friendship level) or conquest (as a result of peace talks). As a result one of the Civs becomes the Liege and the other one becomes the Vassal. The relationship score between the civs continues normally (can be positive or negative).
Main effects:
- If the Liege wins the game, the Vassal also wins the game.
- Liege receives 30% of the Vassal's gold surplus (this can be reduced to 20% or 10% by the liege, which automatically improves the relationship between the Vassal and the Liege), 10% of their food output (spread across the cities) and 5% of their production (spread across the cities).
- Vassal receives 10% of their Liege's cultural output and 10% of their Liege's scientific output.
Diplomatic effects:
- The Liege automatically joins their Vassal's defensive wars if they are attacked by powers outside of their Commonwealth (i.e. the set of civiliziations which are in liege-vassalization relationship between each other).
- Vassals can not declare offensive wars against powers outside of their Commonwealth (i.e. the set of civiliziations which are in liege-vassalization relationship between each other).
- Vassals can declare offensive wars against other Vassals of the same Liege and the Liege must not interfere.
- The Liege can call their Vassal to join Liege's offensive war. The vassal can either join the war (if neutral or positive relationship) or rebel and announce independence (if neutral or negative relationship). If independence is announced, the Liege can declare war on the Vassal or not (allowing the secession to take place).
- The Liege automatically calls their vassals to join Liege's defensive wars. Vassals can accept or refuse to join the war. If they refuse, they automatically announce independence (this is allowed only if their relationship with the Liege is negative).