I'm not sure what the proof is. Although it can be guessed that the Goths of the 5th century were able to lay siege to smaller cities with lesser defensive works perhaps, just after Adrianople they marched on Constantinople and on seeing the city walls promptly turned around realizing they didn't couldn't lay siege to it and later in the early 5th century in Northern Italy they couldn't take Milan in a siege
Alaric's "sack" of Rome, during his fourth "siege", was because the Salarian Gate was opened for him by an inside job of one sort or another. It should be noted that by this time it wasn't even the seat of the emperor, Ravenna was. Rome was undermanned so much so that during one of the earlier sieges by Alaric 6,000 Roman soldiers had tried to fight their way into the city to help man the defensive works.
This is all well after Caesar's time at least, when the Celts were still a power of some degree. Even so, the Goths or any barbarian in these later days for that matter, could potentially hire mercenaries from civilized lands to do the siegecraft for them.