The exact construction date of the castle is currently unknown, but one thing can be confidently stated: it was built in several stages, and every century left its mark on it. What you see before you is the Oltalom (Protection) Bastion, one of the oldest of the castle's 5 known bastions. Do not be deceived by its shape: although it would be more fitting to call it a tower due to its roundness, during the Hungarian-Ottoman wars it was reinforced by being filled with earth, thus perfectly functioning as a bastion, and it survived the centuries in a much better condition than the rest of the stronghold.
What do we know about the earliest periods of the castle? Very little… According to the Gesta Hungarorum, the conquering Hungarian tribes had already subjugated Slavic settlers in Nógrád. Then, around the turn of the millennium, King Stephen I (St. Stephen) made it the center of a county when organizing the county system. However, the castle is not located at or near the geometric center of the historical Nógrád County, just as it isn't for the modern county either: it is situated in the southwestern corner. Let's not forget: in Stephen's time, the center was not Buda, but Esztergom, and starting from there, Nógrád was the first defining stronghold of the county. A similar logic can be discovered in the cases of the formerly neighboring Hont and Heves counties as well.