Next day he examined the position, after having filled up a part of the intrenchment; but Catognatus had done away with the egresses of the northern front, and completed the rampart at this point. The assault was vigorously repulsed. This success emboldened the besieged, and they began to overwhelm the legate with sarcasms. Seeing that he could not take the Oppidum by storm, in presence of a determined and numerous body of men, he resolved upon a regular siege. Although the cutting dug by the defenders between the Oppidum and the plateau was only a bowshot from the rampart, in a few hours it was almost entirely filled up, consolidated, and levelled. Then Titurius had a great number of trees felled in the woods extending along the northern plateau, and brought in front of the camp. This wood being duly prepared, an _agger_ was commenced fifteen paces from the ramparts, in spite of the darts and stones hurled by the besieged. This _agger_ consisted of a terrace about a hundred paces long, ten feet high and twenty deep, with a gap in the middle twelve paces wide. From the two sides of this interval extended at right angles two galleries (_vineæ_), solidly constructed with trunks of trees and covered in; these galleries were about a hundred paces long. The _agger_ was made of trunks of trees piled up, mingled with earth, with inclines for reaching the summit. This was a work of some days; and as during that time the Romans made no attack, and thought only of protecting those engaged in it from the missiles thrown from the ramparts, the besieged did not cease to ridicule them (for they were within hearing), asking them if they were intending to build a city and pass the winter there. But one morning the warriors of the Oppidum saw a wooden tower rising at the extremity of the two galleries. This tower, the woodwork of which had been prepared beforehand, was set up within the day; its summit rose more than ten feet above the top of the towers of the rampart (Fig. 10). [Illustration: FIG. 10.] The Gauls contemplated this structure with uneasiness, although they did not comprehend its importance; so Catognatus resolved to set fire to the works during the night. With this view he had placed on the ramparts, behind the wattling, barrels filled with pitch, grease, and dry sticks; then he placed two bodies in readiness to go out by the eastern and western gates, provided with vessels full of resin, tow, and grease. These troops were to make their way along the outside of the defences, and simultaneously attack the flanks of the besiegers, while the men posted on the ramparts were to remove a portion of the wattling, and to roll the barrels, after setting them on fire, against the _agger_ whose front was raised on the counterscarp of the ditch. [Illustration: FIG. 11.]