This is quite a complicated question to which we do not have a precise answer. The original plan seems to have been for 77 milecastles (MC4 to MC80). Once the ‘forts decision’ was made, however, Milecastles 43 and 80 were replaced by the forts at Great Chesters and Bowness respectively, bringing the total down to 75. The decision to extend the Wall to Wallsend added at least three more (MC1 to MC3) and may have added a fourth (MC0), but that last is speculative. So the total would rise to 78 or 79. However, uncertainty over whether the Wall crossed Burgh Marsh may remove MC74 and MC75 from the total. A final complication is introduced by the fact that a milecastle found in Newcastle is in the ‘wrong’ position and the course of the Wall between Milecastle 3 and the Mining Institute is in places uncertain. Thus the exact number of milecastles is unknown (and possibly even, in the short term at least, unknowable).