The Siege of Jerusalem took place from September 20 to October 2, 1187. It was part of the war called the Third Crusade.
Balian of Ibelin defended Jerusalem against the armies of Saladin. When he surrendered, the Muslims enslaved thousands of Christians but let many leave after they bought their freedom.
A fictionalized version of the siege (attack) is seen in the movie Kingdom of Heaven, directed by Ridley Scott.