Change Main Base
You can transfer your main base to a different castle.
The cost will increase in proportion to the distance from the original location.
Select Council > Change Main Base from the command menu.
Governance Range
Castles outside the governance range of the main base (the daimyō's castle) will have lower incomes.
Try to choose a location that encompasses many castles within your governance range. You could also choose to create new provinces. Moreover, issuing the Postal System policy will expand your governance range.