Which city is known as "The Twin Cities" together with Saint Paul?

Which city is known as "The Emerald City"? 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 Magic City"? Which city is home to the Johns Hopkins Hospital and the Inner Harbor? Which river forms much of the border between Texas and Mexico? Which river forms the border between Oklahoma and Texas for much of its length?