A Traction Fortress is a moving Fortress. They were used by the Nomad Empires' armies to fight in battles and to conquer enemies. The Scriven's Traction Fortress The Barbican was used to invade London. The Movement also used Traction Forts. When London was newly tractionist, Nicola Quercus's enemies attacked London with the help of several Traction Fortresses. However they were defeated by the superior London.They became obsolete by the time of Mortal Engines. It is possible that these where the basis for the later Traction Cities.