'Castle' in the Bible

Then Pekah son of Remaliah, his officer, conspired against him and struck him in Samaria, in the castle of the king’s house with Argob and Arieh; and with him were fifty men of the Gileadites, and he killed him and became king in his place.

A brother offended is harder to be won than a strong city,And contentions are like the bars of a citadel.