Complex sentence: As the 20th century progressed, upscale apartment buildings and smaller houses were added to the neighborhood, although Riverdale continues to maintain its character as an affluent enclave in the city of New York.
Split into simpler sentences: As the 20th century progressed, upscale apartment buildings and smaller houses were added to the neighborhood. To this day, Riverdale continues to maintain its character as an affluent enclave in the city of New York.