A capital is a ruler's primary county.
The location of the capital determines:
- Where the court is located, important when an epidemic disease sweeps through the land
- It is worth more warscore once occupied than other provinces during wars.
- Vassals far from your capital are more likely to seek independence (and vassals bordering the capital duchy cannot create an independence faction)
If affects your army size in several ways:
- Vassal levies
- Demesne levies
- Which county gets the "capital county" bonus to troops.
- Which other demesne counties get the "capital duchy" bonus to troops.
- Which county does NOT suffer from "oversize demesne" penalties to troops.
- Which mercenaries and holy orders are close enough to be conscripted (based on their respective headquarters).
It is also key for many aspects of technology:
- The cultural bonuses that are applied to the whole realm (relations, legalism, ...)
- Retinue size from Military Organization technology
- For merchant republics, the Trade Practices level and thus the number of trade posts each family can build
- The levels shown in the technology screen F5, when not hovering over a province
- Where you spend technology points
- Where your spymaster's "Study Technology" mission improves technology
When you have the ambition to Become King, it determines which kingdom you hope to become king of, and in turn:
- For pagans, which realms you can subjugate.
- Which kingdom title you must gain for success (+1000 prestige) -- otherwise, the ambition is canceled.
Changing the capital
A capital can be changed:
- once per a ruler's lifetime, to any other county within the ruler's demesne (using the button in the top left of the county screen).
- at any time, when moving the capital to the historical location for the primary title (can be seen in the province description on the de jure maps). AI rulers will most often do it.
- for merchant republics, only to the de jure capital of the primary title, although you can switch which of your highest-rank titles is your primary at any time. Make sure your de jure capital is not inland, because an AI doge will move the capital there and destroy the "merchant" part of the republic!
- by granting the current capital county to a vassal. The new capital county will be the one at the top of the list of lands that can be granted, and is typically the oldest held county.
De jure capital
All de jure duchies and kingdoms have a historic de jure capital. Many titular duchies and kingdoms do as well; holding this province is often the sole requirement for creating the titular title.
The AI will often revoke the de jure capital from a vassal, even without justification, in order to move their capital there.
You can confirm a title's de jure capital by clicking the title crown, and seeing that it centers the map in the same location as clicking the province crown.
Select a province with many holdings: more of your direct baron vassals with provide the maximum vassal levies. Your tax base is more certain, especially if gavelkind succession causes you to lose other demesne provinces. If you are rich enough, also consider the number of open holding slots.
Select a technologically advanced province for short term bonuses and spread of technology.
With multiple same-rank titles, choose the county with the largest de jure structure: to maximise levies modifiers, place your capital in the biggest county of the biggest duchy of the biggest kingdom of the biggest empire.
Place the capital at the centre of the realm, to minimize distance penalties and independence factions.