In order to connect the hollows one with another, that an advance or retreat might be made under cover, narrow trenches had been cut at intervals diagonally through the raised mounds of sand. Military experts considered this series of novel fortifications to be practically impregnable, for should the enemy defile through one of the cross passages into a hollow where the Allies were gathered, they could be picked off one by one, as they appeared, and be absolutely annihilated.