Complex sentence: It includes part or all of the Monterey Old Town Historic District, a historic district that includes 17 contributing buildings and was declared a National Historic Landmark in 1970.
Split into simpler sentences: It includes part or all of the Monterey Old Town Historic District, a historic district which is a U.S. National Historic Landmark. The historic district includes 17 contributing buildings and was declared a National Historic Landmark in 1970.