In contrast to other mound centers along the Mississippi, Ohio and Tennessee Rivers, the Spiro site was never fortified by either moat or palisade. Despite their wealth and influence, the Spiroans apparently had little fear of outsiders. Archaeologists assume that they felt secure with their military control of a most strategic site. The Spiro inhabitants depicted themselves as fierce warriors in engraved images on shell cups and gorgets (pendants worn at the throat). It is clear that Spiro was the most powerful of a group of at least 15 political-religious centers in northeast Oklahoma. All of these centers were located at strategic frontier points along navigable waterways in the area, thus allowing Spiroans or their allies to monitor all traders, travelers, or potential enemies coming into their sphere of influence. These northeast Oklahoma natives could easily launch forays into neighboring regions from these sites. Utilizing canoes, parties were sent out to hunt, trade, raid or complete diplomatic missions. Between A.D. 900 and 1350 Spiro was clearly an important political/religious center. It was also the home of artisans who influenced the ideas and works of many southeastern peoples. Conch shell and copper were favored materials for Spiro artisans. They used a variety of techniques including engraving and embossing, depicting elaborate scenes of dance, gaming, warriors, and mythological creatures. Among the latter were winged serpents, antlered serpents, spiders, and catlike monsters that later became important in the mythologies of historic southeastern tribes. At Spiro, however, the animal figures favored by early artisans were later replaced by humanlike figures.