Animal remains (especially cattle bone), domestic waste and pottery have been found at the sites. But there has been limited evidence of any structures. In some locations, such as Windmill Hill, Avebury, evidence of human occupation predates the enclosure. Generally, it appears that the ditches were permitted to silt up, even while the camps were in use, and then re-excavated episodically. It is unlikely that they had a strong defensive purpose. The earthworks may have been designed to keep out wild animals rather than people. The sequential addition of second, third and fourth circuits of banks and ditches may have come about through growing populations adding to the significance of their peoples' monument over time. In some cases, they appear to have evolved into more permanent settlements.