Translation: During the night, while the Trojans are sleeping, the rival soldiers get off the Horse and Troy is attacked.
This sentence takes place after the entry of the Horse in Troy when the surprising attack scheduled by Ulisse is taking place during the night. The Trojans had brought the Horse into the city, thinking that the war between them and the Greek was over. But inside the Horse, there were the most valorous soldiers, led by Ulisse in person.