Complex sentence: In 1804 when the Castle Hill uprising occurred Holt, who was not involved, had been warned that evening that it was about to happen and he set up a defense of Captain Cox's house.
Split into simpler sentences: In 1804 when the Castle Hill uprising occurred Holt, who was not involved, had been warned that evening that it was about to happen. During the night he set up a defense of Captain Cox's house.