SB 10.50.49

iti sammantrya bhagavān
 durgaṁ dvādaśa-yojanam
antaḥ-samudre nagaraṁ
 kṛtsnādbhutam acīkarat
Synonyms: 
iti — thus; sammantrya — consulting; bhagavān — the Supreme Personality of Godhead; durgam — a fortress; dvādaśa-yojanam — twelve yojanas (about one hundred miles); antaḥ — within; samudre — the sea; nagaram — a city; kṛtsna — with everything; adbhutam — wonderful; acīkarat — He had made.
Translation: 
After thus discussing the matter with Balarāma, 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.