A citadel is a strong building or tower inside a city that people used for protection during attacks from enemies. It was usually the strongest and safest place in the city where people could go when they were in danger.
Gideon destroyed the citadel of Penuel after he captured two Midianite kings (Jgs 8:17). Abimelech and his men burned down the citadel of El-berith at Shechem (9:46–49). After, a woman in the citadel at Thebez dropped a millstone on Abimelech's head and crushed his skull (verses 50–54). David conquered Jerusalem by capturing its citadel (2 Samuel 5:7–9; 1 Chronicles 11:5–8).
In the second century BC, during the time when the Maccabees fought for freedom, the citadel in Jerusalem was an important place that helped whoever was in power (1 Maccabees 1:29–33; 11:41–42; 13:49–51). The last major citadel in Jerusalem was called the Fortress of Antonia. The Romans captured it in AD 70.