See CASTLE HILL in Budapest

Castle Hill Sightseeing – The Budapest Castle complex of buildings is on the west side of the Danube river and perched up on Castle Hill overlooking the Danube. On Castle Hill in Budapest, there are many sights – the Vienna Gate, the remains of the Magdalen Tower, the Museum of Military History, the National Archives Buildings, etc. Beneath the Castle Hill are cellars, caves, tunnels and so on cut into the hillside. Parts of the caves have been transformed into a unique and interesting tourist attraction – The Labyrinth. The rest is being slowly excavated – this is the area used as a Bunker by top generals of the Nazi Army towards the end of World War II.
Source: budapestcitytourist.com