western citadel; west wing of Edo Castle
Historical term referring to the western compound of Edo Castle, traditionally the residence of the heir apparent or a retired shogun.
See also: 江戸城
The west wing of Edo Castle was where the shogun's heir or the retired shogun lived.
Compound of 西 (にし, 'west') + の (possessive particle) + 丸 (まる, 'circle' or 'enclosure', here referring to a castle compound). The term is historically specific to Edo Castle.