But all was of no avail. The work, though impeded, still went on. Alexander built large screens of wood upon the pier, covering them with hides, which protected his soldiers from the weapons of the enemy, so that they could carry on their operations safely behind them. By these means the work advanced for some distance further. As it advanced, various structures were erected upon it, especially along the sides and at the end toward the city. These structures consisted of great engines for driving piles, and machines for throwing stones and darts, and towers carried up to a great height, to enable the men to throw stones and heavy weapons down upon the galleys which might attempt to approach them.