THE Aztecs were not gracious victors. Their prisoners of war were frequently used for human sacrifice, as part of spectacles in which their hearts would be ripped from their bodies by priests, to be offered, still beating, to the gods. Their heads fared no better, usually ending up in a kind of skull wall, called a tzompantli in Nahuatl, the Aztec language. In its typical form it consisted of a platform, with posts connected by crossbeams onto which skulls would be threaded. Tzompantlis were generally placed in front of temples, so that friend and foe alike would be awed by the state’s power.

In 2015 archaeologists identified the Huey (Great) Tzompantli, a particularly impressive version. It stood near the main temple of Tenochtitlán, the Aztec capital on whose remains Hernán Cortés founded Mexico City after the Spanish conquest in 1521. As the digging season wrapped up last month, researchers announced their newest discovery: a gruesome, circular tower of skulls, which stood at one end of the 34-metre (100-foot) platform. It is thought to be one of two such towers cited in an account of the Huey Tzompantli by Andrés de Tapia, who fought alongside Cortés.

Today, the tower is around six metres wide. Researchers have uncovered less than two metres in height, but in its heyday, it was probably far taller. The skulls, stuck together with lime and clay, are mostly male, as would be expected of enemy warriors. But others belonged to women and children—groups whose skulls had not been found before on a tzompantli, according to Raúl Barrera, the archaeologist in charge of the excavation.

Even so, their use in the tower ties in with current understanding of certain Aztec ceremonies. Women were sacrificed at feasts and festivals where they were chosen to represent goddesses—on occasion by decapitation followed by flaying. And children were offered to the rain god, Tlaloc, as the tears they shed on the way to their deaths were considered an omen of plentiful rainfall. The adult skulls have holes in the sides, says Mr Barrera, indicating that they were previously displayed on the crossbeams before being moved to the circular tower.

So far, 450 skulls have been identified in the tower. The total in the Huey Tzompantli is likely to be in the thousands. However, the Spanish colonisers probably exaggerated how many they had seen. The squeamish would certainly hope so: de Tapia estimated the number of heads on the crossbeams alone at 136,000.