Celestial City is a location where you can find everything connected with Guilds.
To enter the Celestial City, click on "guild" icon in the lower-left part of the screen.
There are 4 districts in Sky City:
In the "Circle of Summoning", you can summon new titans or receive soul stones of titans you already own. You can make purchases using emeralds or Summoning Spheres, that can be acquired from the Dungeon.
Heart of Power - is the replacement for the old "Forge" with a new feature called "Gift of the Elements"
"Gift of the Elements" allows you to raise your heroes stats for gold and "Sparks of Power" which you receive for making your titans stronger. "Gift of the Elements" has a unique feature to reset these stats and receive all invested sparks (but not the gold) back.
Dungeon - is a new game mode, you will find more information about it in this article.
Titans - is a menu where you can see a list of all owned Titans, raise their level or star rating.