It was necessary at the same time to work at widely extended fortifications, and to fetch in wood and provisions, so that these distant and toilsome expeditions diminished incessantly the effective force of the combatants; and the Gauls, too, often attempted to harass the workmen, and even made vigorous sallies, through several gates at a time. Cæsar judged it necessary to increase the strength of the works, so that they might be defended with a smaller number of men. He ordered trees or large branches to be taken, the extremities of which were sharpened and cut to a point;[526] they were placed in a fosse five feet deep; and, that they might not be torn up, they were tied together at the lower part; the other part, furnished with branches, rose above ground. There were five rows of these, contiguous and interlaced; whoever ventured amongst them would be wounded by their sharp points; they were called _cippi_. In front of these sorts of _abatis_ were dug wolves’ pits (_scrobes_), trunconic fosses, of three feet deep, disposed in the form of a quincunx. In the centre of each hole was planted a round stake, of the thickness of a man’s thigh, hardened in the fire, and pointed at the top; it only rose about four inches above ground. In order to render these stakes firmer, they were surrounded at the base with earth well stamped down; the rest of the excavation was covered with thorns and brushwood, so as to conceal the trap. There were eight rows of holes, three feet distant from each other: they were called _lilies_ (_lilia_), on account of their resemblance to the flower of that name. Lastly, in front of these defences were fixed, level with the ground, stakes of a foot long, to which were fixed irons in the shape of hooks. These kind of caltrops, to which they gave the name of _stimuli_,[527] were placed everywhere, and very near each other. When this work was finished, Cæsar ordered retrenchments to be dug, almost similar, but on the opposite side, in order to resist attacks from the exterior. This line of circumvallation, of fourteen miles in circuit (twenty-one kilomètres), had been formed on the most favourable ground, in conforming to the nature of the locality. If the Gaulish cavalry brought back an army of succour, he sought by these means to prevent it, however numerous it might be, from surrounding the posts established along the circumvallation. In order to avoid the danger which the soldiers would have run in quitting the camps, he ordered that every man should provide himself with provisions and forage for thirty days. Notwithstanding this precaution, the Roman army suffered from want.[528]