The next change was the most radical of all. About 1200 A.D., the people began to desert the mesa tops and within a short time cliff dwellings were built in almost every cave in the Mesa Verde. Shallow caves were available in great numbers for the mesa contains a score of large canyons. The exact number of cliff dwellings is not known but probably there are as many as six or eight hundred in the canyons of the Mesa Verde. The events of the Great Pueblo period seem to indicate that the people were faced with some danger which was not present during the earlier periods. The change from small to large pueblos indicates a need for security and the final move to the caves must indicate a definite need for defense. Hundreds of the caves were high on the cliff faces and many of them were additionally fortified with defensive walls. There can be little doubt that during this period the security of the people was threatened. Now we come to the most difficult question of all. Who was the enemy? Against whom were the people defending their homes? The complete answer is not known but there appear to be two possibilities. It has been suggested that during this period dissension arose within the Pueblo group itself and the people began to war against each other. This theory will be difficult to prove and events of the period seem to argue against it. If the need for defense resulted from trouble within the tribe one might well expect the people to scatter even more widely with groups leaving the heavily populated areas to seek safety in isolation. But the people did just the opposite. Large areas were deserted and the population became more concentrated than in any previous period. [Illustration: Four small cliff dwellings with excellent defensive locations] This drawing together of the Pueblo people may well indicate that the threat was from the outside and it is possible that at this time nomadic Indians entered the area and began to harass the farmers. Ceaseless raids of nomadic marauders would exert tremendous pressure on a farming population and withdrawal from the border lands and concentration in certain favorable areas probably indicates a need for defense against an outside enemy. In 1540, when the Spaniards entered the Southwest, several tribes of nomadic Indians were warring on the Pueblo people and the population had dropped radically. When the Pueblo population was at its height there were hundreds, perhaps thousands, of villages but when the Spaniards came they found less than eighty villages of Pueblo Indians and these were concentrated in a small area in what is now New Mexico and Arizona. Even after the Americans came the population continued to dwindle. Finally there were fewer than thirty villages of Pueblo Indians. Much of this loss of population was due to the harassing activities of nomadic Indians and it is possible that in the Mesa Verde area this trouble began about 1100 A.D.