1.
main enclosure of a castle; castle keep area
The innermost and most heavily fortified area of a Japanese castle, containing the main tower and lord's residence.
本丸に向かう。
Head toward the main enclosure.
本丸の石垣は高くて立派だ。
The stone walls of the main enclosure are tall and impressive.
城の本丸には天守閣がそびえていた。
The castle tower stood towering in the main enclosure.
2.
main target; the heart of the matter; central stronghold
Figuratively, the most important or central part of something; the real target or core issue.
いよいよ本丸に切り込む。
Now we're finally getting to the heart of the matter.
改革の本丸は税制だ。
The core of the reform is the tax system.
捜査はついに本丸に迫った。
The investigation finally closed in on the main target.
本 (main, true) + 丸 (enclosure, bailey). Originally a castle architecture term, now widely used figuratively in politics and journalism.
COMMON COLLOCATIONS:
- 本丸に切り込む: get to the heart of the matter
- 本丸に迫る: close in on the main target
- 改革の本丸: the core of reform
- 本丸御殿: main enclosure palace
SIMILAR WORDS:
- 二の丸: second enclosure — the area surrounding the main enclosure
- 三の丸: third enclosure — the outermost bailey
- 核心: core, crux — a more general word for the central point of an issue
CULTURAL NOTE:
Japanese castles were built with concentric defensive rings. The 本丸 was the last line of defense, containing the lord's quarters and the 天守閣 (castle tower). The figurative use — meaning the real target or the most important objective — is especially common in political reporting.