When you open the game, in the Civilopedia (? in upper right), the top entry is modes, then a description of Dramatic Ages. Worth the read if you haven't. Anything that is not accurate below is my fault for not reading, writing, or testing properly.
Key points to consider:
Only Dark or Golden Ages after first era
No Dedications (like standard mode), but more powerful Policies that can be placed in Wildcard slots.
Earning Era score:
Historic Moments (same as standard mode)
+1 for each Tech/Civic discovered
+1 for each unit promotion to 2d level or higher
Note: Era score is at end of the turn, so
any era score you earn on the turn you transition from one era to the next era is credited to the new era.
Era Score required for Golden age:
Ancient Era -> Classic Era, always 20
If E = Era you are currently entering beyond Classic Era; C = # cities, not including capital; A = +5 for now entering Golden Age, -10 for now entering Dark Age, then:
Era Score needed for next age = (Era score as entering age) + 20 + 5E + 3C + A.
City loses during a Dark age:
In dramatic ages,
you automatically have cities turn into free cities at the beginning of a Dark Age. Note: Free cities have their own influence and enough of them combined can flip other cities
; higher difficulty level, higher influence, 500% on Deity.
At Deity level, human players lose 40% of their cities!!
AI only at 10% at Immortal level. Of note, your capital is always safe and you always lose at least one, but otherwise it is rounded down.
So, if you enter a Dark Age by choice or not, you will lose the following number of cities:
If have 1 (capital only), lose 0. If have 2-4, lose 1. If have 5-7, lose 2. If have 8-9, lose 3. If have 10-12, lose 4...Etc.
