XXX Ulpia Victrix

[Nathan Ross]

AD 394 and the XXX would have been part of Gallic Field Army under the Master of Horse of Gaul.

I would think it very unlikely that the thirtieth survived beyond AD359, when they were last heard of defending Amida. The remnants were probably destroyed with the fall of the city.

The Truncensimani that appear in the ND's list for the Magister Equitum Galliarum are more probably the old Numerus Turnacensium from Britain, as they appear alongside several other ex-'Saxon Shore' units.

If, during the Magnentius rebellion the fortresses along the lower Rhine were abandoned, who was there to protect northern Gaul?

The lower Rhine forts were reoccupied and repaired by Julian in 358-61, but we don't know who formed the garrison. Probably not the original units from before 350; perhaps locally-raised milites formations instead?

By the time of the western section of the ND the lower Rhine appears to have been abandoned north of Cologne (or perhaps even the Mainz district). Stilicho went there in AD396 or so and reorganised the defences, possibly by effectively ceding the frontier to the Franks. They seem to have been the only defenders against the Vandals et al in 405/6 anyway. There may have been Roman troops garrisoned in some of the hinterland cities of Belgica and northern Gaul, however.

Here's a good paper about the situation in the in the 4th and 5th centuries: Soldiers and Society in Late Roman Belgica.