The Betar Fortress was the last standing Jewish fortress in the Bar Kochba revolt of the 2nd century CE, destroyed by the Roman army of Emperor Hadrian in the year 135. According to Jewish tradition, the fortress was breached and destroyed on Tisha B'av, the day of mourning for the destruction of the First and the Second Jewish Temple.
The site of historic Betar (also spelled Beitar or Bethar), next to the modern village of Battir, southwest of Jerusalem, is known as Khirbet al-Yahud, Arabic for "Jewish ruin".
The destruction of Betar in 135 put an end to the last great Jewish revolt against Rome, and effectively quashed any Jewish dreams of freedom. Accounts of the event in Talmudic and Midrashic writings thus reflect and amplify its importance in the Jewish psyche and oral tradition in the subsequent period. The best known is from the Babylonian Talmud, Gittin 57a-b:
"Through the shaft of a litter Bethar was destroyed." It was the custom when a boy was born to plant a cedar tree and when a girl was born to plant a pine tree, and when they married, the tree was cut down and a canopy made of the branches. One day the daughter of the Emperor was passing when the shaft of her litter broke, so they lopped some branches off a cedar tree and brought it to her. The Jews thereupon fell upon them and beat them. They reported to the Emperor that the Jews were rebelling, and he marched against them.
"He hath cut off in fierce anger all the horn of Israel." R. Zera said in the name of R. Abbahu who quoted R. Johanan: These are the eighty thousand battle trumpets which assembled in the city of Bethar, when it was taken and men, women and children were slain in it until their blood ran into the Great Sea . Do you think this was near? It was a whole mil away.
It has been taught: R. Eleazar the Great said: There are two streams in the valley of Yadaim, one running in one direction and one in another, and the Sages estimated that they ran with two parts water to one of blood.
In a Baraitha it has been taught: 'For seven years the gentiles fertilized their vineyards with the blood of Israel without using manure.'
...Rab Judah reported Samuel as saying in the name of Rabban Simeon b. Gamaliel; What is signified by the verse, "Mine eye affecteth my soul, because of all the daughters of my city?" There were four hundred synagogues in the city of Bethar, and in every one were four hundred teachers of children, and each one had under him four hundred pupils, and when the enemy entered there they pierced them with their staves, and when the enemy prevailed and captured them, they wrapped them in their scrolls and burnt them with fire.
Other Midrashic sources can be seen here.