How do you unlock each of the stages?

I'll update this answer as I unlock stages:

Sunken City: Choose "random" at the versus mode stage selection, it will come up eventually.
Towerforge: Beat King's Court in Quest mode on any difficulty.
Moonstone: Play a match on every non-unlockable level in versus mode, then play a match on Sacred Ground.
Ascension: After unlocking the previous 3 levels, go to archives and find the Ascension tab. On each of the 3 stages you can find hidden direction symbols in the background which make a code you can input.
- Final code: Right Up Left - Right Right Down - Down Up Down.

Quest Mode-only Stages:

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