Plutarch Camillus 26

[1] These gave eager welcome to the tidings, so that when Camillus came, he found twenty thousand men already under arms. He collected still more from the allies, and made preparations for his attack. Thus Camillus was chosen dictator for the second time, and proceeding to Veii, he put himself at the head of the soldiers there, and collected more from the allies, with the purpose of attacking the enemy. But in Rome, some of the Barbarians chanced to pass by the spot where Pontius had made his way by night up to the Capitol, and noticed in many places the marks made by his hands and feet in clambering up, and many places also where the plants that grew upon the rocks had been torn away, and the earth displaced. They advised their king of this, and he too came and made inspection.

[2] At the time he said nothing, but when evening came, he assembled the nimblest men and the best mountain-climbers of the Gauls and said to them: The enemy have shown us that there is a way up to them of which we knew not, and one which men can traverse and tread. It would be a great shame for us, after such a beginning as we have made, to fail at the end, and to give the place up as impregnable, when the enemy themselves show us where it can be taken. For where it is easy for one man to approach it, there it will be no difficult matter for many to go one by one, nay, they will support and aid one another greatly in the undertaking. Gifts and honours befitting his valour shall be given to every man.