If it is accepted that there was a walkway along the top of Hadrian’s Wall (and this has long been a matter of debate), then we may justifiably wonder what the top of the curtain wall actually looked like. In 1927, Amédée Forestier offered one possible interpretation.
First, a walkway would almost inevitably demand a breastwork, otherwise it would be pointless. In Forestier’s vision, it is arguably a little on the low side, but we’ll come back to that later. This would need to defend the lower part of the body of anybody patrolling the Wall. There is an extant example of a breastwork at Dura-Europos in Syria, a Hellenized city with a Roman military presence.
The breastwork at Dura-Europos was preserved beneath the earthen rampart the Romans raised to counter the Persian besiegers’ ramp and shows the likely proportion between breastwork and walkway widths (about 1:2), the use of traverses (we shall return to those, too), and the likely form of at least part of the top of the wall (look at the junction with the tower: this reveals that there were merlon caps). However, it must be remembered that the Romans did not build these defences, they merely adopted them when they took over the city in the 2nd century AD.
Now, there is another, rather important, piece of evidence and it comes from the very heart of the Roman Empire: the Praetorian Camp (Castra Praetoria) in Rome itself. The north and east sides of the Castra Praetoria were incorporated in the later Aurelian Walls of the city. As the walls were raised in height over the years (first in the Flavian, then the Severan, and finally the Aurelian periods) the earlier walls were at least partly preserved. This included the very first Castra Praetoria, built in brick-faced concrete under Tiberius. Gate, interval, and corner towers can still be seen, but so can the breastwork. It has traditionally been interpreted as having narrow merlons and broad crenels (below right) but it can equally well be viewed as having broad merlons and narrow crenels (below left).
Eugène Viollet-le-Duc provided an idealised illustration of what a crenellated wall (of any period) looked like.
Time for some terminology:
The surviving walls at Pompeii, like Dura-Europos, demonstrate the use of traverses to enhance the effect of crenellation.
The advantage of traverses was that they served to protect the defender from oblique attack through the neighbouring crenel. The disadvantage was that they took up part of the width of the walkway.
On the one hand, when the defences at Saalburg (Germany) were reconstructed, crenellations with a 1:1 proportion (between crenel and merlon) were used and (uniquely for reconstructions) traverses incorporated. Similarly, for both the Vindolanda and Wallsend lengths of reconstructed curtain wall, a 1:1 proportion was also adopted. On the other hand, the reconstructed length of curtain wall and gateway at South Shields fort were given a proportion of 2:1 in favour of the crenels.
Broad crenels and narrow merlons have a long history in scholars’ views of Roman wall breastworks and two items are key in this interpretation. First there is Trajan’s Column. Many Roman defences are depicted on this and some (but not all) have narrow merlons. Of course, Trajan’s Column is an impressionistic source, in the sense that whilst it can be used to demonstrate that crenellations were used, but cannot be relied upon to get the spacings right: it is not a photographic record.
To reinforce the uncertain nature of Trajan’s Column as a visual source for anything, it is also important to remember that these are turf-and-timber forts and camps that are being depicted, not stone fortifications.
When the defences of the fort at Saalburg in Germany were reconstructed, short traverses were added behind the merlons.
The reconstruction of the stone curtain wall of Hadrian’s Wall at Vindolanda chose not to include traverses. A handrail is de rigeur nowadays, but presumably not in the Roman period.
Similarly, the short reconstructed length of curtain wall at Wallsend also did not include traverses (but once again requires a handrail).
So, what can we say about the top of Hadrian’s Wall? Its width makes a walkway likely and a walkway has to be protected by a breastwork. We know the Romans used crenellations on breastworks, so the Wall would have been crenellated. A hint of this is shown on the Rudge Cup (although even the fact that crenellations are depicted has been disputed: are they in fact turrets which have small merlons on top?).
As for the proportions of the crenellations, logic dictates that the merlons would have been wider than the crenels. Armies, however, are not always logical.
So there we have it. A brief run through the likely appearance of the top of Hadrian’s Wall in the light of what we know from Roman fortifications elsewhere. But is there any physical evidence? Here’s a merlon cap from South Shields, just south of the River Tyne. If they could have them, why not the Wall?
Per Lineam Valli 
Awesome! Thanks for the imagery!
Glad you liked it.