This small, rather dilapidated Mayan ruin on the isolated northeastern shore of Cozumel was probably a guard post, keeping watch over the open sea for enemy ships. Though the Royal Castle - or Castillo Real as it is called, isn't particularly impressive.
You can hike along the ancient coastal road toward Punta Molas, with a working lighthouse. Another set of crumbling Mayan ruins, called Aguada Grande, lies about halfway between.
The dirt road that runs along Cozumel's northeastern coastline to the Royal Castle ruins is in poor shape, and a challenge even in a 4WD vehicle. (Note that your rental car insurance specifically does not cover this road - if you have any problems, you pay everything out of pocket.) Adventurous drivers, however, will be rewarded with beautiful scenery and deserted beaches as they bump and grind their way to the ruins.
Most people visit the Castillo Real by boat, as part of a snorkeling day trip that includes isolated Playa Xhanan.