Missions first, each mission tells you which eras they include, some are multiple eras even all of them. Every mission becomes an endless sandbox once you complete its goals.

Playing in sandbox mode is basically just skipping the game and its stories and going right into the end state where you're already in endless sandbox and didn't have to accomplish anything to get there.

And this isn't really a typical city builder, it's a political sim that happens to let you build cities. Every mission is its own pocket universe.

Originally posted by Kunovega:

And this isn't really a typical city builder, it's a political sim that happens to let you build cities.

This is absolutely true, and I think it's the source of my problem with this series as it's developed over time. It's so ridiculously easy to get reelected now. In the OG T1 it could be really difficult to get reelected, and gaming the system (bribes, assassinations, vote rigging) had much more impact on the population. Now it's not uncommon to hit 100% approval.