Caesar\'s Self-Propelled Siege Tower

[Koyuncu]

Recently while reading the book, The Genius of Archimedes – 23 Centuries ofInfluence on Mathematics, Science and Engineering I met an interesting claim. An article in the book mentions Caesar's usage of siege towers that are moved with no apparent power source. Writers' claim is that it is certain some kind of mechanism used for powering wheels from inside of tower. Yet, I think it is also quite possible to push siege towers within by placing enough human power.

Here the related part of article;

Nevertheless it seems quite certain that the siege towers were self propelled; as for this aspect is concerned, it is also interesting to report the following piece from Julius Caesar (the De Bello Gallico, liber II, par.XXX and XXXI [3]), in which describes the siege at a town of the
Atuatuci Gauls:

XXX – …Ubi vineis actis aggere exstructo turrim procul constitui viderunt,
primum inridere ex muro atque increpitare vocibus, quod tanta machinatio a tanto
spatio instrueretur: quibusnam manibus aut quibus viribus praesertim homines
tantulae staturae - nam plerumque omnibus Gallis prae magnitudine corporum
suorum brevitas nostra contemptui est - tanti oneris turrim in muro posse conlocare
confiderent?

XXXI – Ubi vero moveri et adpropinquare moenibus viderunt, nova atque
inusitata specie commoti legatos ad Caesarem de pace miserunt, qui ad hunc
modum locuti: non se existimare Romanos sine ope divina bellum gerere, qui
tantae altitudinis machinationes tanta celeritate promovere et ex propinquitate
pugnare possent, se suaque omnia eorum potestati permittere dixerunt.

XXX – … As soon as (the Gauls) saw that, having we pushed on the vinea
(mobile roofs) and built an embankment, we started to built a tower, at first they
derided and insulted us because a so big device was built so far (the walls): on
what hands and on what force could ever the Romans rely, small as they were, in
order to bring near the walls a so heavy tower? All the Gauls, in fact, scorn our
height if compared with their large bodies.

XXXI – As they saw that the tower was moved and was approaching their
walls, frightened by the unusual sight, (the Gauls) sent ambassadors to Caesar to
negotiate the peace; they said that they think the Roman make war with the help of
the gods since they can move such big machines so fast, (hence) the put
themselves and all their wealth under the power of Caesar.

So what do you think of this? Is there any other instance or any evidence of such mechanisms for powering siege engines?