ahmus drew his horn and winded a great blast as he stretched his axe at the city , the ranks surged forward with the siege towers in front grinding towards the city while ibis and krocs groaned as they pushed , arrows rained on the attackers from the wall , many fell only to be trodden by the iron wheels of the towers , a rattle of impaling arrows could be heard clanging upon the towers of dark steel like the clatter of raindrops in a storm .