stronghold; inner citadel; bastion
Often used figuratively for an opponent's or enemy's base of power, especially in political, business, or competitive contexts. The literal meaning refers to the innermost fortified part of a castle.
敵の牙城を攻める作戦が立てられた。
A plan was made to attack the enemy's stronghold.
彼は長年、政界の牙城を守ってきた。
He has long defended his bastion in the political world.
From Chinese 牙城, the fortified headquarters marked by a commander’s 牙旗, or command flag. It consequently developed the senses “inner citadel” and “stronghold.”