sentences:  Then, they set up a solid defensive system and besieged the citadel.Besides the military tactics, Tây Sơn commanders applied a psychological tactic that made the former Tây Sơn soldiers who were now serving in Nguyễn forces to defect and return to the Tây Sơn army.The tactic succeeded, forcing Võ Tánh to seal the citadel's door and wait for reinforcements.