The towering flames had now surmounted every obstruction, and rose to the evening skies one huge and burning beacon, seen far and wide through the adjacent country. Tower after tower crashed down, with blazing roof and rafter; and the combatants were driven from the court-yard. The vanquished, of whom very few remained, scattered and escaped into the neighbouring wood. The victors, assembling in large bands, gazed with wonder, not unmixed with fear, upon the flames, in which their own ranks and arms glanced dusky red.
-Sir Walter Scott, Ivanhoe
Yet fire was not only a siege weapon but a daily tool in such castles: the main form of heat, and the only form of light beyond the sky.
(Via Medievalists.net, which has far more video from the story.)