1/6. 1179: the Crusaders build a new - and supposedly impregnable - fortress at a crossing on the Jordan called Jacob's Ford. But, within a year of its completion, the castle, which was protected by 1500 Knights Templar, was taken in a siege lasting just five days. The historical strand returns with Dr Thomas Asbridge trying to solve the mystery of why the stronghold fell so quickly. Michael Praed narrates.