What did Wellington do when the French set fire to the chateau at Hougomont? Why did he do this?

When the French set fire to a chateau at Hougoumont, Wellington took time to pen a note of instruction to his subordinate commander. The note instructed the soldiers to stay near the burning chateau as long as they were out of danger. After the fire died down, the men could take up arms inside the burnt out chateau. It was also a maneuver that would keep the French from doing the same thing.