The importance of long-distance alignments in Roman surveying is increasingly being recognised. It has now been discovered that they were used in setting out the central sectors of the Antonine Wall, but — in contrast to Hadrian's Wall — it appears that they were employed to determine the locations of the military installations along the Wall rather than the line of its rampart and ditch. It also appears that the enigmatic enclosures and expansions which are attached to the rear of the rampart of the Wall only seem to occur in connection with these alignments. A careful analysis of possible explanations indicates that the Romans may initially have sought to set up a two-level alarm system across the central sectors of the Antonine Wall, the possible impacts of which upon the planning and design of the Wall are examined. The Supplementary Material available online (https://doi.org/10.1017/S0068113X18000284) contains a table of inter-visabilities between known military installations, and OS grid references for the installations.