The years from 1100 to 1300 were Mesa Verde's classic period. The population may have reached several thousand. It was mostly concentrated in compact villages of many rooms, often with the kivas built inside the enclosing walls rather than out in the open. Round towers began to appear, and there was a rising level of craftsmanship in masonry work, pottery, weaving, jewelry, and even tool-making.
The stone walls of the large pueblos are regarded as the finest ever built in Mesa Verde, they are made of carefully shaped stones laid up in straight courses. Baskets show evidence of decline in workmanship, but this may be due to the widespread use of pottery and consequent less attention to the craft.
About 1200 there was another major population shift. The Anasazi began to move back into the cliff alcoves that had sheltered their ancestors long centuries before. We don't know why they made this move. Perhaps it was for defense; perhaps the caves offered better protection from the elements; perhaps there were religious or psychological reasons. Whatever the reason or combination of reasons, it gave rise to the cliff dwellings for which Mesa Verde is famous.