Crenelations are the indentations atop the battlements of a fortification. They enhanced its defensive capability so much that, in the medieval period in England, one needed a ‘licence to crenelate’ from the monarch. A crenelated parapet enables a defender to shelter behind the merlon (the higher part) and shoot or cast through the crenel (the gap between neighbouring merlons).
Roman crenelations are depicted on Trajan’s Column and survive embedded in the walls of the Castra Praetoria in Rome, as well as on the city walls of Dura-Europos in Syria. Other archaeological finds indicating their use (notably merlon caps and parapet footings) have been made in Britain, from the legionary fortress at Chester. No indisputable archaeological finds are known from the Wall, although the Rudge Cup has a stylised crenelated design on top of what appear to be turrets.
Scholars have long argued that Hadrian’s Wall was not used as a fighting platform and so the issue of the existence or not of crenellations on it has been a contentious one. The most important point to note about crenellations is that, if they were indeed used on Hadrian’s Wall, they imply the existence of a walkway.