Castle

The Castle is the beating heart of your Estate. Upgrade to increase your  March Capacity  and unlock new buildings.

Info
The info sub-menu will show you the building Bonuses. The first number is the one based on the building level, and the second one (in green) is based on other bonuses (such as Research, Talent points, etc.)  "More Info" will show you an expanded view of the below table, along with the Power  generated by each level.

When the Castle is not being upgraded, there is an additional button under the Info menu that allows you to swap the other Urban and Rural buildings (within the same category) for 200 gold pieces.

In addition to the standard sub-menus, you will see the following building specific sub-menus:

Estate Stats
Details production of each resource and estate upkeep, by bonus source, and individual resource building.

Appearance
Allows you to purchase and use castle skins.

Estate Buffs
Allows you to view and use estate buffs by category.

Coat of Arms
Allows you to view and embed Coat of Arms by building.

Reference: