The castle in Troy was called the Pergamos, what was kept there and why was it so important?
The story told about the Palladium: It is said that when Athena (a Greek goddess) was born she was brought up by Triton, who had a daughter named Pallas. Both girls practised the arts of war, but one time they had a fall out, and when Pallas was about to throw a punch, Zeus in fear interfered with the fight, and Pallas being startled, looked up, and fell wounded by Athena. Grieving for Pallas, Athena made a wooden image in her memory, and set it up beside Zeus and honoured it. But afterwards Electra hid the statue and Zeus threw the Palladium into the Ilian country and Ilus(Ilus was Troy) built a temple for it, and respected it. The reason why it was so important was because Troy couldn't fall unless the Palladium had fallen and been taken out of the city.
(Picture above is Ajax, one of the best soldiers, trying to dragging Cassandra, a Trojan princess, from the Palladium)