Icon under a microscope

This landmark is iconic and easily recognisable when you see the whole picture, but can you identify it from this small portion of the image?

This structure is part of the world’s largest art museum and is a central landmark in the city where it is found.

Parts of the museum were built as a palace in the 12th and 13th centuries, but this section pictured was added in the 1980s and is now the main entrance.