There are more than 40 km's of natural tufa caves underneath castle hill, which have been used for everything from housing for homo erectus cavemen 500,000 years ago, to bomb shelters for 20,000 Buda citizens during WW II. In the early 20th century, the caves were extensively modernized and contain a self-contained hospital and command center for the Hungarian government. At that time, many of the natural entrances and exits were bricked off. Today you can take a tour of parts of the "labarinths" by lantern-light, which was quite interesting and taught me a bit of Hungarian history.