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The most comprehensive visual evidence about Roman army of I-II centuries is the Trajan column. It shows in great detail cloths, weapons, ships, military machines and other equipment of the Roman army. Here is an enigmatic depiction from this column:

click for the picture

The image is taken from this web site.

It seems from context that this is some siege equipment. What could this possibly be?

This actually came up in discussion when I was studying the archaeology of Roman Britain.

In fact, as far as I'm aware, nobody knows for certain, but several archaeologists that I have spoken to suggested that they were meant to be three-wheeled plutei, which were screens intended to protect the attacking force besieging a city.

The objects can be seen here in context against the walls of the city:

• Image source Wikimedia
  

Plutei and other forms of protective shield are described in Vegetius' Epitome of Military Science (pp128-129). In fact, in the linked edition, in the footnotes on page 129 the editor, N.P Milner, notes that:

The triangular chassis with three wheels which are illustrated on Trajan's column, Cichorius Pl. LXXXV scene cxiv, may be intended to represent plutei.

It is worth noting that the archaeologist Ian Richmond, then at the British School at Rome, was of a different opinion. He did not consider that the objects were part of the Roman siege equipment at all.

In his paper Trajan's Army on Trajan's Column, published in Papers of the British School at Rome Vol. 13 (1935), pp. 1-40, he discusses the features of the objects and concludes that it more likely that the objects represented festucae, or 'pounders' used in the construction of the Murus Gallicus, and as such, would have been used by the Dacian defenders, rather than the Romans attacking the city of Sarmisegetusa.

Such devices are mentioned by Vitruvius in Book 3 of his De architectura where he describes the construction of temple foundations:

"Intervalla autem concamaranda aut solidanda festucationibus, uti distineantur"

translation:

"The spaces between the columns are to be arched over, or made solid by being rammed down, so that the columns may be held apart."

Professor Richmond went on to argue that:

"... their presence here will indicate that this part of the fortification was newly completed, shortly before the arrival of the Romans."

This interpretation has the advantage that it does at least explain all the visible features of the objects but, as I said earlier, it is not possible to be certain.

It seems to be not entirely clear. But it also seems to represent an unusual depiction of an otherwise well known instrument:

Scene of a siege from Trajan's Column (Rome). The defenders roll barrels and tree trunks downhill (top right), but these are intercepted by three peculiar machines. The German scholar Otto Lendle made
the astute observation that these are likely, to represent Apollodorus' 'ship's prow tortoises'. (C. Cichorius, Die Reliefs der Traianssaule, Berlin 1900)

Duncan B Campbell: "Besieged. Siege Warfare in the Ancient World", Osprey: Oxford, New York, 2006, p197.

File:085 Conrad Cichorius, Die Reliefs der Traianssäule, Tafel LXXXV (Ausschnitt 01).jpg:

In the original called the Schiffsbugschildkröten of Apollodorus. If true then it would be one of his Poliorketika, another form of testudo or (chelone).

Personally, I'd been lead to belive this looks most likely like a crossbreed between an early trebuchet and some kind a wrecking ball, although both are not recognised at such an early date.