Durán notes that the were periodically renovated. Regarding this, Durán states"When skulls become old and deteriorated, they fell in pieces. When the palisade become old, however, it was renovated, and on its removal many broke. Others were removed to make room for more, so that there would be a place for those were to be killed later"Archeologist Eduardo Contreras believes that the tissue attached to skulls was removed prior to a pole being inserted through the side of the skulls. He bases these assumptions off of excavations of the Plaza de las Tres Culturas at Tlatelolco, Mexico City between 1960 and 1965.