It really is nothing more than some caves hollowed out of the cliff. Years later, a five-story masonry construction was built higher up on the cliff wall. Supposedly about 45 people lived in the "apartment house" before mysteriously fleeing the area, never to return.
The ruins are now off-limits to tourists although a park is operated at the base of the cliff and thousands of visitors come to the park annually. I've been there several times but this week was a perfect time to visit with temperatures around 70 degrees.
Incidentally, the name "Montezuma's Castle" is a misnomer. When the ruins were discovered by European Americans in the 1860's, they were believed to have been built by followers of the Aztec emperor of Mexico. In truth, he never came anywhere near this far north. But the name stuck.