Which city is known as "The Emerald City"?

Which city is known as "Music City"? Which city is known as "Sin City"? Which city is known as "The Big Apple"? Which city is known as "Beantown"? Which city is known as "The City of Angels"? Which city is known as "The Gateway to the West"? Which city is known as "Steel City"? Which city is known as "The Twin Cities" together with Saint Paul?