The Maya-Toltec ruins of Tulum are perched on coastal cliffs in a beautiful setting above the azure sea. The ruins are 12th century, with city walls of white stone. The temples were dedicated to the worship of the Falling God, or the Setting Sun, represented as a falling character over nearly all the west-facing doors. (Cozumel was the home of the Rising Sun). The same idea is reflected in the buildings, which are wider at the top than at the bottom.
The main structure is the Castillo, which commands a view of both the sea and the forested Quintana Roo lowlands stretching westwards. All the Castillo’s openings face west, as do most, but not all, of the doorways at Tulum. Look for the alignment of the Falling God on the temple of that name (to the left of the Castillo) with the pillar and the back door in the House of the Chultun (the nearest building in the centre group to the entrance). The majority of the main structures are roped off so that you cannot climb the Castillo, nor get close to the surviving frescoes, especially on the Temple of the Frescoes.
Tulum is very popular - take a towel and swimsuit if you wish to scramble down from the ruins to one of the two beaches for a swim (the larger of the two is less easy to get to). The reef is from 600 m to 1km from the shore, so if you wish to snorkel there you will need to take a boat trip.