inner citadel; main enclosure
Historical term for the innermost, most fortified part of a Japanese castle. Still used in historical contexts and as a metaphor.
The inner citadel of Himeji Castle is very spacious.
The enemy army finally attacked the castle's inner citadel.
Figurative extension of the castle term: the most important part, the heart of a matter, or the main focus. Often used in business, politics, or strategy.
The core of this project is the development of the new product.
The crux of the discussion was the budget issue.
The soldiers defended the inner citadel to the very end.
The investigation finally began closing in on the core of the case.
In this negotiation, the pricing issue is the real crux of the matter.
中心 is a neutral word for 'center' or 'middle', whereas 本丸 specifically implies the most important, most defended part of something.
要 highlights the indispensable pivot point of something, while 本丸 frames that same point as the ultimate target or stronghold to be reached.
本丸 is the standard castle-terminology word for the innermost bailey, whereas 金城 is a more literary or figurative term for the same inner stronghold.
本丸 is the innermost, most important enclosure of a castle, while 二の丸 is the secondary enclosure just outside it, so the two are easy to mix up in castle contexts.
本丸 refers to the whole innermost walled enclosure of a castle, while 天守閣 is specifically the tall keep tower that usually stands inside it.
From 本 (hon, 'main') + 丸 (maru, a suffix used in castle names and enclosures). Originally referred to the main bailey or innermost compound of a Japanese castle.