The fortlets on the Wall are very important for understanding both how it functioned and was planned. There are, however, substantial gaps in our knowledge of these installations.
Until Gillam’s seminal paper (1975) prompted a search for more, only four fortlets were known on the Wall, at Duntocher, Wilderness Plantation, Watling Lodge and Glasgow Bridge. Five further fortlets were then discovered relatively quickly at Kinneil, Seabegs Wood, Cleddans, Croy Hill and Summerston, though the evidence for the latter is quite slight (Keppie and Walker 1981; Hanson and Maxwell 1986, 93–5; Maxwell and Hanson 2020). Three others have been suggested; however, there is insufficient evidence to be confident about fortlets at Rough Castle and Laurieston (cf Macdonald 1933 with respect to Rough Castle), but resistivity data provides support for the identification of a fortlet at Castlehill (Hanson and Jones 2020). Most recently a section of the rampart of what is probably another fortlet has been uncovered during a rescue excavation ahead of a house extension at Boclair. Where it has been possible to test the relationship, all fortlets were either contemporary with or preceded the construction of the linear barrier and so must be seen as part of the original design (Hanson and Maxwell 1986, 93; Hanson 2020b). However, the consistency of this evidence in respect of Wilderness Plantation and Kinneil has been challenged (Symonds 2017, 139).
Gillam hypothesised that originally the Antonine Wall fortlets were positioned at approximately one-mile intervals, like the milecastles on Hadrian’s Wall. He further suggested that they were interspersed between six primary forts, but that some 50% of the fortlets were subsequently removed when further forts were added to the Wall during the construction process (Gillam 1975; see Planning and building the Wall). This model met with general agreement. Keppie and Walker put forward a list of 36 fortlets interspersed between primary forts (1981, 161), while Hanson and Maxwell suggested that there may have been as many as 41 (1986, 122). The most detailed list of fortlets was provided by Woolliscroft, who included grid references, the distances between suggested locations and a note on the evidence (1996, 160 and 167). The situation was considered so fluid, however, that he actually provided alternative lists, one with 39 and the other with 41 sites. A further list based on the analysis of the LiDAR data reasserts a potential list of 41 sites emphasising the consistency of spacing one Roman mile apart (Hannon et al 2020).
Various considerations have resulted in recent challenges to Gillam’s overall scheme. These include the failure to find more fortlets after the initial successes; the irregularity of the spacing of those discovered; apparent inconsistencies in the structural relationships between the Wall and some of the supposed secondary forts; and assertions about the primacy of all fort locations in relation to the planning of the Wall line (Poulter 2009, 117–24; Graafstal et al 2015; Symonds 2017, 144–6). Nonetheless, it remains the case that fortlets lie too close to forts at Duntocher and Croy Hill for them to be easily seen as part of the same unitary plan. However, unless more are found, the existence of a full cordon of fortlets located every mile will remain open to question. If past assumptions about the regularity of their spacing have been too prescriptive, the proactive search for further examples will be more difficult. Nonetheless, two more have been found in recent months by geophysical survey undertaken by HES: one at Carleith in a predicted location, but a second at Bonnyside only some 300m west of the fort at Rough Castle. However, in neither case has the detailed evidence yet been published or the wider implications assessed.
Releasing the Wall fortlets from a fixed-spacing system, in which their position in relation to the landscape was essentially arbitrary, makes an assessment of their topographical setting potentially more informative. Indeed, it is clear that the independent fortlets beyond the west end of the Wall at Lurg Moor and Outerwards were carefully placed within the landscape to oversee the western coastal flank of the Wall. Some on the Wall line, such as Duntocher, may have controlled concealed access routes, while there are indications that the westernmost fortlets may have been sited in order to oversee the terrain to the south of the Wall, which is potentially significant for understanding its role (Symonds 2017, 144–9).
It is widely agreed that the design of the fortlets was heavily influenced by that of the milecastles on Hadrian’s Wall (for example Hanson and Maxwell 1986, 93–5; Robertson 2015, 27; Breeze 2006, 86). The most celebrated feature of the latter, the presence of paired gateways providing access through the barrier, is also integral to the former. As on the earlier turf built sections of Hadrians Wall, more postholes seem to be present at the north gateways than at the south, though both sets should have been sufficient to support a tower. A causeway over the Wall ditch opposite a fortlet is known only at Watling Lodge where the main road to the north crossed the frontier line (Breeze 1974, 166). The apparent change in the width of the ditch at Kinneil suggests that a crossing point may originally have existed and was subsequently dug out, as on Hadrian’s Wall (Bailey and Cannel 1996, 337; Welfare 2000). However, recent geophysical survey seems to indicate that there was no reduction in ditch width, though the interpretation of the data continues to be disputed (Bailey 2021, 218-20; Hanson et al forthcoming). If the majority of the causeways had been eliminated during the operational lifespan of the fortlets, this may suggest that there were fewer places to cross the Antonine Wall over time.
One curious feature is that the northern rampart of each fortlet constructed as one with the Wall seems to have been wider than the other three, presumably to facilitate the seamless integration of the fortlet with the Wall. However, this suggests that the rampart around the rest of the fortlet would have been lower. The evidence from the two fortlets at Duntocher and Cleddans, erected before the Wall, is less clear on this issue. The presence of a defensive ditch or ditches, not usually provided outside the milecastles on Hadrian’s Wall, is a significant addition to the design, possibly linked to the absence of an equivalent to the vallum on Hadrian’s Wall.
Too little is known about the internal layout of the fortlets to be certain that a standardised approach was adopted. However, in the three whose interiors have been more extensively explored, wooden structures, presumably barracks, have been recorded on both sides of the internal road. The fortlet at Kinneil also has evidence for a lean-to building set against the northern rampart and a well (Robertson 1957, 16–33; Wilkes 1974, 55–7; Bailey and Cannel 1996, 310–14 and 336–41). Slight traces of structures on one side of the central road were also noted at Croy Hill (Hanson 2022 a, 23–27).
Three sites (Kinneil, Seabegs and Croy Hill) have also provided evidence of features just outside the rampart that relate to their occupation. We are no more confident of the nature of the fortlets’ garrisons, whence they were derived or what function they were intended to perform, than we are for the milecastles on Hadrian’s Wall, though presumably one function was to control movement.
How and when the fortlets were abandoned is disputed. A secondary layer of cobbling apparently sealing the interiors has been recorded within all the fortlets that were sufficiently well preserved for it to survive (Robertson 1957, 23–7; Wilkes 1974, 57 and fig 2; Bailey and Cannel 1996, 315 and 342–4; Hanson 2022 a, 27–8). This has generally been interpreted as indicating that the fortlets were decommissioned during the life of the Wall and has been linked with slight signs that the gateways at Seabegs and Kinneil may have been narrowed or removed. However, attention has been drawn to the pottery from these two sites that could indicate a longer period of occupation (Keppie and Walker 1981, 149; Bailey and Cannel 1996, 329). In that case, the cobbling may simply have been intended to provide a useful hard surface in damp conditions (Symonds 2017, 142–4).
- Establish whether there was a consistent spacing pattern, and whether there are more fortlets to be found.
- Undertake further geophysics at key locations to ascertain if structures such as fortlets can be identified.
- Undertake further analysis of the position of known fortlets with reference to the physical and human geography of the area.
- Further analyse the material culture and structural evidence from any excavated fortlets in order to assess the character and history of their occupation and abandonment.
- Establish, where possible, the relationship between fortlets and other elements of the Wall system.
- Locate, review and publish data from the re-excavation of the fortlet site at Duntocher undertaken in 1978.
- Take any opportunities to examine areas around fortlets to establish whether there is any associated activity.