Late Roman Unit Sizes

[Nathan Ross]

I was just having a look at some of the recorded sizes of late Roman fortifications, and trying to work out the possible numbers of men they could have housed (this is following Richard Duncan Jones in Structure and Scale in the Roman Economy, and Brewer's Birthday of the Eagle, mostly). This seems a reasonable way to try and calculate the possible size ranges of various limitanei units, and perhaps other units too.

Troesmis, main base of II Herculia, is 2.8 hectares. The legion also has subunits ('cohorts') at three other locations, one of which, Axiopolis, is 0.6 hectares.

Noviodunum, up the Danube, is 5.8 hectares, while El Lejjun is 4.6 hectares and the 'Camp of Diocletian' at Palmyra is 4 hectares. All of these, like Troesmis, appear to have been the main bases of Diocletianic frontier legions. Richborough, where II Augusta are based in the ND, is 2.4 hectares, while Divitia (Deutz) on the Rhine is 2.25. 

Then there are a lot of very small fortifications, listed in the ND as housing a cohort - Kellmunz at 0.86 hectares, Eining at 0.18 and Burgle at 0.16.

So how many men could actually be accommodated at these places?

The Principiate legion fortresses at Caerleon and Inchtuthil were presumably built to house c.6000 men - one is 20 and the other 20.5 hectares. So maybe we could assume a rough estimate of c.300 men per hectare, assuming the density of housing, stores and other buildings was about the same.

That would give c.840 men at Troesmis, c.1740 at Noviodunum, c.1380 at El Lejjun, c.1200 at Palmyra, c.720 at Richborough, and c.675 at Deutz.

The smaller 'cohort' forts have very small garrisons - c.258 men at Kellmunz, only c.54 at Eining and c.48 at Burgle.

Taking the Troesmis garrison, though: we know that II Herculia at one time had 10 cohorts (inscription from North Africa, c.AD298). The ND has three 'cohort' subunits away from base, presumably leaving 7 at Troesmis itself. Divide the 840 men there by 7 and we get 120.

The subunit base at Axiopolis would house 180 men - but the ND also has the Milites Superventores based there as well. If the latter unit was only 60 men strong, the legion 'cohort' could indeed be 120 men. At Troesmis we also have the Milites Secundi Constantini, however, who do mess up the calculations a bit!

But could the Legion II Herculia originally have been raised 1200-strong, with ten 120-man 'cohorts'? That would make a new 'cohort' roughly the size of an old maniple. The limitanei legions further up the Danube, split into five 'cohort'-sized subunits, would be 600 men strong when united, and the field army detachments of these legions would therefore be 600 men strong as well (five 120-man 'cohorts')

Both El Lejjun and Palmyra, meanwhile, look like they could house 1200 men quite comfortably - probably the undivided Legions IV Martia and I Illyricorum respectively.

The small limitanei forts could easily house multiples and divisions of a 'cohort' - 240 at Kellmunz, perhaps, with 60 each at Eining and Burgle.

There is the problem of Legio I Iovia at Noviodunum, which has two 'cohorts' out-stationed, and despite sharing its base with the Milites Primi Constantini would appear to have about twice as much space as it would need, if it really was the same size as the legion at Troesmis! And why are Richborough and Deutz so small by comparison?...

So perhaps there's nothing in all these calculations after all!