castle of a feudal lord; lord's residence castle
Refers specifically to the castle where a feudal lord (daimyō) lived and governed, as opposed to a branch castle or a castle used only for military purposes.
This castle was once the residence castle of a Sengoku-period daimyō.
城主は居城を拡張して城下町を整備した。
The lord expanded his residence castle and developed the castle town.