 Complete the following phrase:  Trenches continued to be built around the city until they surrounded the entire perimeter by March 1343. Behind the trenches were earthen banks, and on these wooden walls were erected to protect the besieging soldiers, with strong towers erected at intervals. Trebuchets in the Castilian camp hurled large numbers of stone balls, or bolaños. The trebuchets had a maximum range of , and were vulnerable to parties of besiegers that were able to cross the trenches. So many bolaños were launched during the siege that in 1487 King Ferdid II of Aragon sent an expedition to the ruins of Algeciras to retrieve them so they could be used again in the 