Eustace, as a member of the council held at Ruj on 4 January 1099, mediated the conflict over the control of Antioch between Bohemund of Taranto and Raymond IV of Toulouse. In early December 1098, Eustace joined Raymond's attack on Maarrat al-Nu'man and an attack on Nablus in July 1099. He gained notoriety for his actions during the siege of Jerusalem fighting relentlessly from a siege tower along with his brother Godfrey and the crusaders they commanded. They were among the first to breach Jerusalem's city walls and participated in the ensuing massacre. Eustace commanded a division of the crusader army during the Battle of Ascalon, and was a patron of the Knights Templar.