These preparations rendered it clear to Clodoald that the enemy since his first checks was acting with method, and preparing for a decisive action. He had quickly perceived that his attack would be directed to the weak points of the fortress,--that is to say, the northern salient and the banks of the river opposite the western bend of the cité; he had therefore strongly barricaded all the roads of the town leading to the quay, and had strengthened the latter with a _vallum_. [Illustration: FIG. 23] In addition to this, two hundred paces behind the square tower on the river, to the north, he had run another vallum, _a_, _b_, through the houses and gardens, following the slopes of the plateau in an oblique direction, and joining the south-west gate (Fig. 23). The habitations had been left as a mask in front of this entrenchment; a few houses and fences only had been cleared away to give a free space outside. [Illustration: FIG. 24.] Clodoald could not attempt anything before the northern salient, the enemy being there in front of him in force; but within the salient itself he sunk ditches with retrenchments of earth and stakes, as shown in Fig. 24. These works being low and masked were invisible to the enemy outside. Every night he sent out of the city, by the postern which led to the bottom of the wide fosse on the northern front, spies who rendered him an account of the operations of the enemy. At the end of the third week from the beginning of the siege, his spies reported a considerable degree of activity in the large camp; that faggots were being got ready, that the soldiers were preparing their arms, and that war-engines were being mounted. One of these spies, who crossed the river below the town and observed the attitude of the enemy encamped on the west, brought a similar report. Clodoald judged, therefore, that the besiegers were on the point of attempting a grand effort on the west and the north. On the morning of the twenty-third day of the siege, in fact, four _onagri_ planted on the work opposite the small _tête de pont_ swept the latter with stones so effectually that the defenders were scarcely sheltered behind the parapets, and could not work the engines placed at that point. At the same time, boats laden with inflammable materials were launched in the river above the wooden bridge. These boats, impelled in the direction required, were arrested by the piles of the bridge, and were not long in setting it on fire (Fig. 25). The defenders of the small _tête de pont_, seeing that their retreat was going to be cut off, abandoned the work, which was soon occupied by the Franks. Retired within the _place d'armes_ behind the bridge, the besieged could do nothing but watch the fire. [Illustration: FIG 25.]