Another possible cause is invasion by outside peoples, though the only evidence of warfare found are the defensive wooden stockade and watchtowers that enclosed Cahokia's main ceremonial precinct. There is no other evidence for warfare, so the palisade may have been more for ritual or formal separation than for military purposes. Diseases transmitted among the large, dense urban population are another possible cause of decline. Many theories since the late 20th century propose conquest-induced political collapse as the primary reason for Cahokia's abandonment.