The building, nowadays called the Piast Tower, dates back to the 14th century and is the only entirely preserved defensive tower of the Upper Castle. It was built on a square plan out of split sandstone and stone blocks. Under the tower, there are partly filled-in cellars, 6 m high, which supposedly used to be the beginning of a secret passage leading outside the castle walls. In the Middle Ages, it was one of the four towers forming the defensive walls of the castle. It acquired its present shape of a freestanding building during the construction of the Habsburg Palace in the 19th century. In the same period, a clock was installed inside the tower, whose mechanism has survived until today. The tower is 29 m high, its walls are 2 m thick, and there are 120 stairs running up the top. Originally, the tower had a wooden canopy crowned by a tent-like shingled roof with two spires. The corner bay windows are adorned with escutcheons presenting the image of the Piast eagle, symbolising the ducal power; they were created in the workshop of master Piotr Parléř from Prague.