The Citadel is the modern name for a ruin whose walls were built with primitive mortar and local stones straight up from the edge of the top of the rock, a remarkable achievement. Archeologists wonder why the Anasazi often built in high, hard-to-get-at places, with theories ranging from defense to avoiding croplands, gaining sun and breeze, or simply choosing a dramatic and beautiful hilltop setting. Behind it, a large circular sink is a natural depression in limestone, formed when cracks let rainwater dissolve the rock and collapse material above it, and such sinks do not hold water. Beyond the sink rise the San Francisco Peaks, volcanic mountains that are the highest point in Arizona and are considered sacred by modern day Hopi and Navajo people.