From that webpage:
Lol. Yeah, of course a battering ram isn't going to knock down a sturdy stone wall. That was true the world over in the 3rd, 2nd, and 1st centuries BC. Celtic oppida are not exceptions to the rule. Rammed earth and stone fortifications can still be destroyed by heavily concentrated artillery fire from pieces such as the lithoblos, palintone, ballista, and onager. If you can knock down a stone wall in Syria, you can knock down a stone wall in Gaul; one is not inherently more special than the other. These walls were built more than a thousand years before the existence of thicker walls like those in 16th-18th century "star forts" designed with low, thick, angled walls to resist gunpowder artillery barrages. As far as I know ancient Celts didn't build anything remotely as defensible or strong as a star fort, so I see this entire claim by the EB II team as incredibly dubious without further information.
In Jeff Kinard's
Artillery: an Illustrated History of its Impact, he states that the Roman onager, originating as early as the 3rd century BC (although he makes it clear the Romans were slow in adopting artillery from the Greeks and Carthaginians), was able to hurl stone projectiles up to a few hundred yards (a 100 lb could be hurled 400 yards), each shot weighing up to 180 lbs (i.e. 82 kg). Even the less imposing artillery on the Roman side, the ballista (the two-handed torsion artillery piece described by Vitruvius in the late 1st century BC, in the time of Augustus), was able to fire stone ammunition weighing 60 lbs (27 kg) up to 550 yards, traveling at a speed of 115 miles per hour (185 kilometers per hour). The Jewish Roman historian Josephus tells us about the destructive power of the ballistae on both men and
stone (!!!) fortifications during the siege of Jotapata in 67 AD (pp 16-17):
This was the effect of the ballista being fired; you're telling me that the lithobolos and onager (now available for recruitment in in EB II), with stone ammunition that was twice as heavy, couldn't knock down Celtic stone walls with enough heavy concentrated fire? Let's look at an example of a Celtiberian
castro (hill fort), the San Cibrao de Las
oppidum of northern Portugal. You think Roman artillery, which was damaging the stone walls of Jotapata, was incapable of penetrating this?