Introduced in the 1.05 patch, Assimilation is the process by which the de jure borders of kingdoms and empires can change over time. In general, a duchy will assimilate into a kingdom after being controlled by that kingdom for 100 years, and a kingdom will similarly assimilate into an empire in the same time frame.
Every duchy in the game is considered to be part of a specific de jure kingdom. For example, in 1066, Flanders is part of the de jure kingdom of Frisia, but is entirely controlled by the King of France. If France controls Flanders for long enough, Flanders will eventually be considered part of the de jure territory of France. This will make the local rulers of Flanders regard the French king more favorably, and will also make it easier for France to reclaim the territory if it is conquered by another realm or declares independence.
Assimilation requires all of the following conditions to be met:
- The entire duchy is within the realm of the king.
- The king does not hold the crown of the current de jure kingdom to which the duchy belongs.
- The ducal title either does not exist or is held by a vassal of the king.
- The duchy is not part of the kingdom of Jerusalem
If this is the case the duchy will, after one year, appear shaded with the colors of the assimilating kingdom (blue over orange, in this case) on the de jure kingdoms screen. Hovering over the duchy will show a tooltip indicating the number of years until full assimilation.
Whenever the conditions for assimilation are not met - for example, if the local ruler of Flanders rebels against France, or if one of the counties in Flanders is conquered by the Holy Roman Empire - the counter will tick backwards instead of forwards. When the conditions are again met - for example, if the duke's rebellion is put down - the counter will resume ticking forwards.
Kingdoms assimilate into empires in an identical fashion - the emperor must hold all territory in the de jure kingdom.
Assimilation allows titular kingdoms and empires to become non-titular. It can also potentially completely eliminate a de jure kingdom (the de jure kingdom would, at that point, become a titular kingdom).
Assimilation can be both good and bad from a gameplay perspective - an assimilated Duke becomes more loyal and can vote in elections, but the assimilated Duchy no longer counts towards the total territories needed to claim a crown. For example, suppose as the King of Scotland you control the Duchy of Ulster in northern Ireland. Before the duchy assimilates, you control 4/13 counties in Ireland - you'd only need 3 more to proclaim yourself the Irish King. But if Ulster assimilates into Scotland, you'll now have 0/9 counties in Ireland and need to conquer 5 more to become King of Ireland.
As of patch 1.08 kingdoms no longer assimilate out of de jure empires that have no holder.