Oliver’s education suited him to write accounts of the campaign in the form of letters back to Europe, encouraging recruiting there. He reworked his letters into detailed accounts of the Fifth Crusade that became the Historia Damiatina, a source used by many historians since. As a historian, Oliver wrote about the Holy Land and accounts of earlier Crusades, using the work of famous chroniclers Fulcher of Chartres and William of Tyre.
Far from being just an observer or religious leader, Oliver of Paderborn is most famous for designing and directing construction of the siege tower in the summer of 1218, which allowed the crusaders to take over the city of Damietta. The tower was built on top of two ships lashed together, topped by a structure of wood, ropes, and leather. From this tower, the crusader army was able to mount ladders to scale the walls of the city and invade it. He wrote:
We joined two ships which we bound together sturdily by beams and ropes.…We erected four masts and the same number of sail yards, setting up on the summit a strong fortress joined with poles and a network fortification. We covered it with skins about its circumference…and over its top as a defense against Greek fire. Under the fortress was made a ladder, hung by very strong ropes and stretching out thirty cubits beyond the prow.
After the defeat of the Crusade at Damietta, Oliver continued his work of promoting and recruiting, especially for the Crusade of Emperor Frederick II (1227–1229). He had written letters to the ulama (Muslim religious scholars) of Egypt, and he was also interested in encouraging missions to Eastern rite Christians, in support of papal efforts, and he advocated for military orders such as the Teutonic Order of knights to crusade to spread Christianity in the Baltic region of northern Europe. Oliver was elected bishop of Paderborn in April 1225, and then became a cardinal.
Source: “Oliver of Paderborn (d. 1227) – The Crusades.” Accessed December 21, 2017. https://erenow.com/postclassical/crusades/695; Sandra Alvarez. “Oliver of Paderborn and His Siege Engine at Damietta » De Re Militari.” http://deremilitari.org/2014/04/oliver-of-paderborn-and-his-siege-engine-at-damietta/.