Complex sentence: The castle has required continuing maintenance work, in particular to its roof--itself an important historical feature; efforts have been made to gradually redecorate the castle in a period style, using reproductions of original wallpapers and materials.
Split into simpler sentences: The castle has required continuing maintenance work, in particular to its roof, itself an important historical feature. Efforts have been made to gradually redecorate the castle in a period style, using reproductions of original wallpapers and materials.