The last stronghold of Bar Kochba's rebels, which fell on the 9th of Av, 135 CE. Here is one of the stories about the fall of Beitar.
Hadrian besieged the fortress of Beitar for three years. Finally, he decided to end the siege. A pagan came to him and said, "My lord, wait, I will arrange for you to capture Beitar today." The pagan approached the gates of the fortress, found Rabbi Elazar praying near them, and pretended to whisper something in his ear.
They went and reported to Bar Kochba that Bar Kochba's uncle, Rabbi Elazar, was planning to betray the fortress of Beitar to Hadrian. Bar Kochba ordered the pagan to be brought to him and asked him, "What did you whisper in Rabbi Elazar's ear? What did he tell you?" "If I tell, the emperor will kill me. If I don't tell, you will kill me. I would rather destroy myself than reveal the emperor's secrets." Then Bar Kochba ordered Rabbi Elazar to be brought and asked him, "What did this pagan tell you?" Rabbi Elazar replied, "I don't know what he whispered to me. I was praying and heard nothing." Bar Kochba became enraged, struck Rabbi Elazar, and killed him. That same day, the fortress of Beitar was captured, and Bar Kochba was killed. Eighty thousand soldiers entered Beitar and killed almost every man, woman, and child.