The ANBU Headquarters, from ground level, appears to be a two story building. It's built not far away from the Hokage tower, against the stone wall. However, most people don't realize that the building leads to a cave system that is used for most of the ANBU's secret work. Prison cells and interrogation rooms are far down in the network of caves, making escape virtually impossible. When you walk in the doors you are greeted by a plain reception room with no signage, and it is almost always empty, save a single secretary. The remainder of the building, both above and below-ground, is strictly off limits to civilians.