SB 10.50.49
iti sammantrya bhagavan
durgam dvadasa-yojanam
antah-samudre nagaram
krtsnadbhutam acikarat
Translation:
After thus discussing the matter with Balarama, the Supreme Personality of Godhead had a fortress twelve yojanas in circumference built within the sea. Inside that fort He had a city built containing all kinds of wonderful things.