From 1989 to 1996, excavation and surveys were carried out at dozens of sites and intersite areas in the southwestern Peten by the Defensive Systems Subproject of the Vanderbilt Petexbatun Regional Archaeological Project and by subsequent related Vanderbilt investigations. The excavations and analyses explored fortification systems, related settlement, and artifactual evidence. Beginning at about a.d. 760, the major centers of the Classic Maya civilization in the Petexbatun region were fortified by a massive expenditure of labor on defensive walls of masonry, usually surmounted by wooden palisades. As warfare accelerated, major centers and later even small hilltop villages were located in highly defensible positions and were fortified by walls, palisades, moats, and baffled gateways. Despite these efforts, all major centers were virtually abandoned by the early ninth century. By a.d. 830, only the island fortress of Punta de Chimino and a very reduced and scattered population remained in the Petexbatun region.