Indicated by its well-defended location, Tulum arose during the Post-Classic Period (the Classic Period is regarded as extending between A.D. 300-900). It thrived as a center of seagoing trade from about A.D. 1200 until the arrival of the Spanish. There have been many archeological investigations, beginning in 1848 (see "Incidents Of Travel In The Yucatan" by John L. Stephens and Frederick Catherwood). This incredibly picturesque site (now overrun daily by visitors disgorged from tour buses) comprises 16 acres, featuring small, clustered, eroded buildings, facing west. Oriented to the four cardinal points. Watchtowers were situated on each corner of the western wall. Most murals and sculptured reliefs have eroded away. Mockingbirds and Great Kiskadees call from the rooftops.