The Emerald City is the capital city in L. Frank Baum's fictional world 'The Land of Oz' from the Oz books.
"A bright greenish glow filled the sky above the city in the horizon. A massive gate had been built around the city and was covered in emerald and jewels that twinkled so brightly in the sun, it was almost blinding."
Alex is very disappointed they can't go inside.
- ↑ TLOS IV, ch 15, p. 201