Gallorum eadem atque Belgarum oppugnatio est haec. ubi circumiecta multitudine hominum totis moenibus undique in murum lapides iaci coepti sunt murusque defensoribus nudatus est, testudine facta portas succedunt murumque subruunt. quod tum facile fiebat. nam cum tanta multitudo lapides ac tela conicerent, in muro consistendi potestas erat nulli.
(Caesar, De Bello Gallico 2.6.2-3)
The Gauls and the Belgae use one method of attack. A host of men is set all round the ramparts, and when a rain of stones from all sides upon the wall has begun, and the wall is stripped of defenders, the attackers form a “tortoise”, move up to the gates, and undercut the wall. This was easily done on the present occasion; for when so vast a host hurled stones and darts, no man might stand firm on the wall.
(tr. H.J. Edwards)
Crambe Repetita 