Complex sentence: It's monumental staircase and it's architecture of astonishing modernity for the time do of him one of the major monuments of the end of XVIIIth centure and a reference for the appreciators of neoclassicism.
Split into simpler sentences: It's monumental staircase and it's architecture are of an astonishing modernity for the time. It is one of the major monuments of the end of XVIIIth century and a reference for the appreciators of neoclassicism.