Primitive man defended his community from the ravages of his enemies by a thick thorn hedge or a wooden palisade. But a spear could penetrate these meagre defences, and the next stage in the race of better defences to ward off methods of attack, better methods of attack to penetrate the defences, and so on, was to build mounds of earth to ward off the enemy's spears. When the defenders realized that they needed defences which the enemy could not climb over, they made their mounds relatively higher by digging a ditch round the outside. Thus increased in height, the mound kept off the enemy for a time, but before long they in turn had learned to fill up the ditch with earth and faggots and thus to neutralize the depth of the ditch. So the wall had to be made higher, and as there is a limit beyond which one cannot pile up a mound of earth, the mound was strengthened with logs. The enemy had a counter-measure to this too, and made a breach in the defences by setting fire to the logs which formed the basis of the mound. This incendiarism was checked for a time by piling layers of earth and hides on the logs and so placing the logs that they could not be got at easily by the attackers. But this did not prove satisfactory, and we come at last to the wall of masonry or brick as the best means of defence. To this basic element many additions and improvements were made—the defenders learned, for instance, to make their perimeter curved as opposed to angular, because it was easier for the enemy to undermine a corner than a curved stretch of wall; and they learned to build towers out from the walls from which they could give flanking fire against the enemy who reached the bottom of the wall, at whom those on the main part of the wall could not fire with impunity.