About Adventist Health Castle. Medical center serving all of O’ahu. A full-service medical center offering a wide range of inpatient, outpatient and home-based services, Adventist Health Castle is a 160-bed facility with more than 1,000 employees and staffed by more than 300 physicians. Located just outside of Kailua, Adventist Health Castle serves all of O’ahu and is the primary healthcare facility for the Windward side of the island. Adventist Health Castle is owned and operated by Adventist Health, a Seventh-day Adventist healthcare system.