As late as the 17th October Wellington had no fears about Hill’s position. The probability that the enemy from Valencia would advance upon Madrid, as he wrote on that morning, seemed diminishing day by day--reinforcements were coming up from the South to Hill, and Ballasteros was believed to be already posted in La Mancha, ‘which renders the enemy’s movement upon the Tagus very improbable[79].’ On the 19th, when his last assault on the Castle of Burgos had just been beaten off, and he was thinking of retreat, came much less satisfactory news from Hill. The enemy seemed to be drawing together: they had laid siege to the Castle of Chinchilla, and it was not impossible that they were aiming at Madrid, perhaps only with a view of forcing the abandonment of the siege of Burgos, perhaps with serious offensive intention. If so, Wellington might be caught with not one but both of the halves of his army exposed to immediate attack by a superior enemy, who was just assuming the offensive. And the total force of the French was appalling--50,000 men on the Northern field of operations, 60,000 on the other--while the united strength of all the available troops under Wellington’s orders was about 80,000, of whom 25,000 were Spaniards, whose previous record was not in many cases reassuring. He had often warned the ministers at home that if the French evacuated outlying provinces, and collected in great masses, they were too many for him[80]. But that ever-possible event had been so long delayed, that Wellington had gone on, with a healthy and cheerful opportunism, facing the actual situation of affairs without taking too much thought for the morrow. Now the conjunction least to be desired appeared to be at length coming into practical existence and it remained to be seen what could be done. As for his own army, there was no doubt that prompt retreat, first beyond the Pisuerga, then beyond the Douro, was necessary. Souham had already made his first preliminary forward movement. On the evening of October 18th a strong advance guard, consisting of a brigade of Maucune’s division, pushed in upon the front of the allied army at the village of Santa Olalla, and surprised there the outlying picket of an officer and thirty men of the Brunswick-Oels, who were nearly all taken prisoners. The heights above this place commanded the town of Monasterio, wherefore Wellington directed it to be evacuated, and drew in its line to a position slightly nearer Burgos, for it was clear that there were heavy forces behind the brigade which had formed the French attacking force[81].