An ingeniator was probably a military engineer, responsible for the king's artillery. They are first recorded in Domesday Book, though given the importance of fortifications in England since the reign of Alfred the Great, it seems improbable that siege engines and military engineers were introduced at the Conquest. That said, all the engineers recorded in Domesday Book were clearly of continental origin.
For further information on engineers, see Jim Bradbury, The medieval siege (1992); and for royal servants, J.H. Round, The king's sergeants and officers of state with their coronation services (1911); E.G. Kimball, Sergeantry tenure in medieval England (1936); and R.R. Darlington, 'Introduction to the Wiltshire Domesday', Victoria History of the county of Wiltshire, vol. 2, edited by R.B. Pugh and E. Crittall (1955), pages 42-177.