British squares prepare to face French cavalry
By this point in the battle, Napoleon appears to have become preoccupied with the threat of the Prussians to his right. There was a lull in the fighting (except around Hougoumont), during which time Wellington reinforced his line with the last of his British reserve troops and the French artillery prepared to launch another bombardment. This bombardment began at about 3pm. At 3:30pm, Ney ordered an assault on La Haye Sainte. This was repulsed but Ney noticed the British line falling back to shelter behind the crest of the ridge and mistook this for a general retreat. He ordered forward a curassier brigade to harry the Allied line and then more cavalry. When the cavalry arrived at the Allied line, they found well-formed infantry squares rather than a retreating rabble. There followed a series of massed cavalry charges, Led in person by Ney, and containing some 5000 horsemen across a 700 yard front between Hougoumont and La Haye Sainte. Each time, the British gunners pounded the advancing French cavalry until the very last minute and then retreated into the shelter of the squares. By 5pm, the French cavalry were exhausted and the Allied line remained intact. By this point, the threat from the Prussians was becoming more acute and Napoleon had to send Lobau's Fourth Corps and the Imperial Young Guard to fend them off long enough to defeat Wellington.
At 6pm, Ney again personally led an assault on the Allied line, this time at the head of the remainder of Reille's infantry. This attack was also fairly unsuccessful, but soon afterwards he managed to gain control of La Haye Sainte. This was vital for the French because it allowed their artillery to move forward and really threaten the Allied position. Ney seems to have realised that Wellington's situation at this point was desperate and he pleaded with Napoleon to advance the only remaining French reserve, the Old Guard, to complete the victory. Napoleon was reluctant to release his beloved elite troops and hesitated for half an hour. This gave Wellington enough time to reinforce his centre using troops from the left flank which was now safely in the hands of the advancing Prussians. By the time Napoleon gave the order to send the Old Guard forward, the Allied line had regained some semblance of order and was in a far better position to receive an attack than it had been when Ney first requested the troops.