At the fall of Teotihuacán the mountains in the central plateau saw the construction of several city-states protected by walls, including Xochicalco, one of the most dynamic and a Unesco World Heritage Site. It stands on top of a levelled hill at the crossroads of several trade routes. In addition to its elegant Feathered Serpent Temple and its enchanting setting, Xochicalco (the "place of the house of flowers") also boasts an underground astronomical observatory.