The large sentient robots were built by the pepole of the flying city of Laputa.
One of the Robots fell from the city and was hevialy damaged however it was discoverd by the milatary, when they captured the young girl Sheeta the magic from her necklace managed to revive the robot that went on a rampage around the army fort but when it saw Sheena in trouble it coverd her from any danger as she was royal Laputan blood, sacrifeing itself to save her from missile fire.
Later when Sheena and her freind Pazu manage to land on the castle they meet the last running robot on Laputa albeit coverd in vines and leaves and it shows them the Tree of life directly in the center of Laputa, it acted as a gaurdian for all the animals on the city.
Later when Colonel Muska stole the necklace and took over Laputa he used the robots to kill as many of his troops as possible then he used them to bomb the ground and go on rampages around the towns, however when Muska was killed the robots ceased their rampage.