Complex sentence: The original windows have been replaced, probably in the early 19th century for the upper windows to the north and the 20th century for those to the south, while the ground floor windows are modern but within the original openings.
Split into simpler sentences: The original windows have been replaced; that to the north probably in the early 19th century, that to the south in the 20th. The ground floor windows are modern but within the original openings.