He was to have no easy task. Caswallon was a foeman worthy of Cæsar’s steel. He seems to have fully realized that the British foot-levies were useless against the legions except at a great advantage. He declined to defend the line of the Stour, but as soon as it was across the stream, the Roman column found itself engaged in a running fight, in which the heavily armed and burdened legionaries were at a disadvantage. The next day Caswallon set upon the Romans as they were laying out their camp, drove in the outposts, killing the tribune in command, burst through the intervals of the supporting cohorts, and drew off with little loss, after causing great confusion. This half success, however, led to a severe defeat. Probably the king could not restrain his eager and undisciplined foot-levies. On the morrow Cæsar sent out all his cavalry and three legions under Trebonius to forage. The force was attacked by the British mounted arms, but the foot-levies got out of hand and charged the legions while they were still in close order. They were repulsed with much bloodshed, of course, and the Roman cavalry, closely supported by infantry, was able to cut them up badly before they could regain the woods. Caswallon, unshaken, fell back on his guerrilla tactics. His chariotry and cavalry were intact. Cæsar says that they were about four thousand, a moderate estimate which shows it to be near the truth. With them he faced his great antagonist, while the southern levies took refuge in the Weald, and those of the Catuvellauni went back to entrench the fords of the Thames. From the neighbourhood of Canterbury to that of London the Romans advanced slowly, probably along the line of the later Watling Street, while Caswallon moved parallel to them in the woods, hung about the line of march, and harassed it incessantly. The Gallic horsemen never dared to move far from the infantry columns. Caswallon was ever watchful. But it was not guerrilla warfare, however skilful and gallant, that could stay the march of Cæsar’s legions. The advance was slow and difficult, but still it continued steadily until the Thames was reached. Caswallon retreated across it, and the Romans followed.