Complex sentence: The first house was begun in 1910 and finished in 1911; by the beginning of the 21st century, Fieldston, a gated community, was one of the wealthiest neighborhoods in New York City.
Split into simpler sentences: The first house was begun in 1910 and finished in 1911. By the beginning of the 21st century, Fieldston, a gated community, was one of the wealthiest neighborhoods in New York City.