Castillo Real is a partially excavated site with a temple, two chambers, and a lookout tower. It is believed to have been a Maya watchtower to protect against approaching enemies.
It’s located on the remote northeastern corner of the island [1], along the sand road leading to Punta Molas.
Not only is the road itself quite treacherous, it’s often closed due to a land dispute there. Ask at the tourist office for the latest before making any plans to head up there.
Links:
[1] http://www.moon.com/destinations/cancun-the-yucatan/cancun-cozumel-and-the-riviera-maya/isla-cozumel/sights/beaches-and-beach-clubs/northeastern-coz