III. Now, first of all, after the Cimbri and Teutones had invaded Gaul, he was serving under Cæpio[109] at the time when the Romans were defeated and put to flight; and, though he lost his horse and was wounded in the body, he crossed the Rhone swimming in his cuirass and with his shield against the powerful stream--so strong was his body and disciplined by exercise. On a second occasion, when the same barbarians were advancing with many thousand men and dreadful threats, so that for a Roman to stand to his ranks at such a time, and to obey his general, was a great matter, Marius had the command, and Sertorius undertook to be a spy upon the enemy. Putting on a Celtic dress, and making himself master of the most ordinary expressions of the language, for the purpose of conversation when occasion might offer, he mingled with the barbarians, and, either by his own eyes or by inquiry, learning all that was important to know, he returned to Marius. For this he obtained the prize of merit; and in the rest of the campaign, having given many proofs of his judgment and daring, he was honoured and trusted by his general. After the close of the war with the Cimbri and Teutones, he was sent as tribune by Didius[110] the prætor to Iberia, and he wintered in Castlo,[111] a city of the Celtiberi. The soldiers, being in the midst of abundance, lost all discipline, and were generally drunk, which brought them into contempt with the barbarians, who, by night, sent for aid from their neighbours the Gyrisœni, and, coming on the soldiers in their lodgings, began to slaughter them. Sertorius with a few others stole out, and, collecting the soldiers who made their escape, surrounded the city. Finding the gates open through which the barbarians had secretly entered, he did not make the same mistake that they did, but he set a watch there, and, hemming in the city on all sides, he massacred every man who was of age to bear arms. When the massacre was over, he ordered all his soldiers to lay down their own armour and dress, and, putting on those of the barbarians, to follow him to the city from which the men came who had fallen on them in the night. The barbarians were deceived by the armour, and he found the gates open, and a number of men expecting to meet friends and fellow-citizens, returning from a successful expedition. Accordingly, most of them were killed by the Romans near the gates, and the rest surrendered and were sold as slaves.