then the balrogs con- tinued to shoot darts of fire and flaming arrows like small snakes into the sky , and these fell upon the roofs and gardens of gondolin till all the trees were scorched , and the flowers and grass burned up , and the whiteness of those walls and colonnades was blackened and seared : yet a worse matter was it that a company of those demons climbed upon the coils of the serpents of iron and thence loosed unceasingly from their bows and slings till a fire began to burn in the city to the back of the main army of the defenders .