Ksiaz is a castle in Walbrzych. It was built in 1288-1292 under Bolko I the Strict. It lies within a protected area called Ksiaz Landscape Park
Since 1509, the building has been owned by the Hoberg family, known later as the Hochberg family. Ksiaz Castle, along with its vast estates, remained in the hands of the Hochberg family until World War II, when it was confiscated by the Nazis. In 1939 the Nazi state took, on the pretext of alleged treason, as a result of the family's English connections, most of the estates belonging to the Hochberg family.
In 1941, the Nazis finally took over the castle and began it's reconstruction by command of the Fuehrer. At the same time several thousand prisoners were digging a tunnel network within the framework of the “Riese” Project, which also was the reason for the mineshafts being dug in the Owl Mountains. However, before the work was completed, Germany was forced to surrender. Part of the underground section of the complex at Ksiaz Castle are two hollow excavated chambers, located 15m and 50m below the main courtyard. The first chamber, that is open to the public, is fully concreted and has a total length of about 90m at the level of the Goddess Flora Terrace and the adjoining terrace. It connects the castle directly with the terraces and the main shaft. It has a guarded sentry post, and exit on the Horseshoe Terrace and is connected to one of the castle lifts, which allows communication with all the floors of the castle, as well as the nearly circular staircase in cross-section, to ensure communication with the ground floor of the castle.
The second excavated chamber is much more extensive and has been used for mounting super-sensitive measuring instrumentation of the Geophysical Observatory of the Polish Academy of Sciences.
The Ksiaz Castle and Palace Complex is a historic building, which is listed as a historic monument as part of the Ksiaz Landscape Park.