Milecastles are usually reconstructed as having towers (see figure above), although most such reconstructions only opt for a northern tower (above, centre). However, in all cases where a milecastle has been examined, the north and south gateway foundations are identical, so it would seem logical that both were equally equipped: either they both had towers (above, right) or neither did (above, left). The logic for the existence of the towers is that the one Turf Wall milecastle examined in detail, MC50TW, had more posts than could just be accounted for in a gateway, and so it was felt that it must have had a tower. Likewise the elevation provided by the turrets would be wasted if the intervening milecastles did not share it. All towers in other contemporary Roman fortifications, whether built as part of a fort or free-standing on a frontier, were square. If the milecastles had towers, they were of a unique form.
Further reading: Symonds and Mason 2009