a fortified citadel or palace-complex: the walled royal and administrative stronghold at the center of a capital
A birah is a fortified stronghold on the grand scale: the walled citadel or acropolis at the heart of a capital, where the king's throne, treasury, and garrison sat together behind heavy defenses. It is more than a residence and more than a fort; it is the whole royal and administrative complex, built to impress and to withstand a siege.
Most often the word names "Susa the citadel," the fortified upper city of the Persian capital, kept distinct from the lower town where ordinary people lived. This is the stage for the book of Esther and for Daniel's vision, and it is where Nehemiah served the king. Nehemiah also uses the word closer to home, for the fortress that guarded the Jerusalem temple and for the citadel of the city itself.
Chronicles makes the boldest move: it calls the temple a citadel-palace, insisting that this stronghold "is not for man, but for the LORD God" (1 Chronicles 29:1). The temple is pictured as the royal fortress of the divine King, as grand and as guarded as any imperial acropolis. Related Hebrew words divide the ground differently: הֵיכָל names a palace or temple as a building or great hall, while אַרְמוֹן names a fortified stronghold in the older prophets.