We got another developer diary from the new Paradox game Millennia!
This time the diary is about the ages in Millennia. The age system is somewhat comparable to Civ3, as there are defined ages, which can be unlocked by more things than only technologies. The first player entering a certain age will have the option to chose from a historical age, from an alternative timeline age (e.g. steam punk age), or they might trigger a crisis age (like probably the dark ages). If a player enters one of the first two, they will stay locked for everyone, but you can get out of a crisis age by triggering one of the others. After each alternative age you have to play a historical age though, to keep the timeline a bit on track.
For some more insights please check this thread.