Sea City

Sea City is a city located in the east.

The Lizard Man Heroes Arc
Among the countries and cities listed in the New World's incomplete map by Ainz Ooal Gown, Sea City happens to be one of them alongside Eryuentiu.

The Invaders of the Large Tomb Arc
Sea City is mentioned by Platinum Dragon Lord during his discussion with Rigrit Bers Caurau concerning who or which person he should ask for help. He stated that the high chance of waking some unknown female individual up from her slumber in Sea City is a better choice to do than requesting aid from his fellow Dragon Lords.

Layout
Platinum Dragon Lord stated to Rigrit that in the deepest level of the Sea City lies an unknown female individual sleeping who he seems to be aware of for quite some time now. This may hint that there might be a layer of levels for one must transverse through in the Sea City to reach the deepest part of it all.

Trivia

 * Based on Platinum Dragon Lord's response to Rigrit, it seems possible that the person he was hoping to seek aid from against Players is most likely located in Sea City.