A flat-topped mound dominating the western side of the plaza was the site of an important structure, either the ruler's dwelling or a religious building. The mound stands 15 feet high, measures 138 by 120 feet at the base, and 95 by 75 feet at the top. In 1877, when Frederic W. Putnam of the Harvard Museum excavated a trench there, he found that the mound had been built in three phases, with ash covering the two lower levels. Its construction was a communal undertaking requiring community organization, an understanding of geometry, and knowledge of building techniques. Some mounds were built in a single event, while others grew through repeated episodes as older structures were replaced by newer ones, and excavated mounds can show color variations from individual baskets of dirt or stratified layers of different soils or sands that may reflect either added construction layers or ritual intent. The village at Sellars was enclosed by a complex palisade with a three- to four-foot-high outer earthen wall, a three- to four-foot-deep ditch, and a four- to five-foot-high inner wall topped by wooden poles, probably red cedar. Three entrances lay on the southern and southwestern sides, where earthen causeways crossed the ditch. Projecting earthen bastions set about every 110 feet along the palisade served as guard towers for defense, and gardens of corn, beans, and squash lay outside the palisade.