Milecastle 0

Milecastle 0 is a possible milecastle of the Roman Hadrian's Wall which may have preexisted the fort of Segedunum. Although its existence has been suggested by historian Peter Hill, no evidence of this milecastle has been found. It is not known whether the decision to establish forts on the line of the wall predated the decision to extend the wall to Wallsend, so it is possible that this milecastle was never built. No excavations or investigations have taken place specifically for Milecastle 0 and no evidence has ever been found during excavation of Segedunum.

Source: Wikipedia