Also in the central precinct is the Callo, which is tall. The Castillo was built on a previous building that was colonnaded and had a beam and mortar roof. The lintels in the upper rooms have serpent motifs carved into them. The construction of the Castillo appears to have taken place in stages. A small shrine appears to have been used as a beacon for incoming canoes. This shrine marks a break in the barrier reef that is opposite the site. Here there is a cove and landing beach in a break in the sea cliffs that would have been perfect for trading canoes coming in. This characteristic of the site may be one of the reasons the Maya founded the city of Tulum exactly here, as Tulum later became a prominent trading port during the late Postclassic.