Of all the cities of Mesoamerican civilizations, Tulum is the only one built on the coast and the only one fortified with a wall. It was the first Mayan city spotted by the Spanish in the 16th century and paradoxically the last Mayan fortress during the conquest. The structure called the Castle was used as a lighthouse to guide the Spanish galleons away from the reef as they attempted to come ashore. It resembles a limestone cliff overlooking the beautiful turquoise waters of the Caribbean Sea. Also the Temple of the Frescoes, which still bears traces of the original color in its murals, gives the visitor a clear view of this civilization's complex writing system and cultural achievements.