noun
Refers to a fortified place or a secure base, often in a military or historical context.
敵は山の上の根城に立てこもった。
The enemy holed up in their stronghold on the mountain.
noun
base of operations; headquarters
Extended use for a place where someone or a group is based, such as a gang's hideout or a company's main office.
ここが彼らの活動の根城だ。
This is their base of operations.