As soon as Marlborough saw that the defence of Tournai was weakening, he marched a large force towards the lines of La Bassée; again demonstrated against de Villars, whom he puzzled completely, and then, after a march of forty-nine miles in fifty-six hours, in pouring rain over roads knee-deep in mud, swooped upon Mons, a town important to the French, though at that time only weakly held. When de Villars slowly realised that the Duke had no intention of wasting his strength in storming highly fortified lines, he advanced with 95,000 men, and entrenched himself at the Trouée d’Aulnoit, one of the few gaps in a belt of woodland which lay a few miles to the south of Mons. He outnumbered the Allies so greatly that they decided not to attack him until the troops left to level the siege-works at Tournai had rejoined headquarters, and thus made Marlborough’s numerical strength equal to that of his opponent. These troops, among which were the XVIIIth and several other British regiments, were ordered up at once, but while they were on the march de Villars, by working night and day, rendered his position very formidable. His right rested on the forest of Laignières, half a mile from the village of Malplaquet,[59] which has given its name to the battle of the 11th of September, 1709: his centre ran across the southern end of the gap, which was open, fairly level, and about 2000 yards in width: his left was thrown forward into the continuous series of woods known respectively as those of Taignières, Blangies, and Sart. Across his centre he built long lines of trenches, gun emplacements, chains of abattis, and many strong redans; the woods on his flanks were similarly protected, and when the attackers had forced their way through the abattis which fringed the edges they came under the fire of field-works hidden in the depths of the forest. The weak point of the position was that cavalry could only act offensively on the plain--_i.e._, the gap between the woods; and as this open ground was covered with defences Villars had to draw up his Horse in rear of the rest of his troops, where they would be unable to come into action unless the Allies broke through some part of the front line.