Complex sentence: On the evening of 4 March 1804, a hut at Castle Hill was set afire as the signal for the rebellion to begin which was not seen by the convicts at Green Hills, today's Windsor, on the Hawkesbury River.
Split into simpler sentences: On the evening of 4 March 1804, a hut at Castle Hill was set afire as the signal for the rebellion to begin. This fire was not seen by the convicts at Green Hills, today's Windsor, on the Hawkesbury River.