I took a few days to explore the editor, and it turned out to be easier than I thought to restrict the game to certain eras. I did some playtesting during my experiment.
In my little project, I stayed in Ancient Times. I just deleted the more advanced units, improvements, advances, etc. The results were interesting.
First of all, there are some balance issues. Since the most powerful universal unit is the swordsman in Ancient Times, iron is at an absolute premium. In a standard game you can overcome a lack of iron with gunpowder or advanced horse units. Only when you get to railroads does iron become critical again.
You can remove the prerequisite of iron for swordsman in the editor, because, without swordsmen, you'll have a hard time taking territory to secure an iron resource deep in enemy territory. You kinda lack offensive umph. You don't want to remove the swordsman from the game because it makes spearmen defending a city almost invincible.
Then, there are the Immortals and Legionaries, both super-swordsmen. This gives Persia and Rome a military advantage, but not insurmountable.
Another surprise is when my Persian opponent began making a railroad in about 400 AD. With the restriction to Ancient Times, civs only need iron as a prerequisite for railroads, since steam power was deleted as an advancement, and therefore as a possible prerequisite.
I'm still working with it. Perhaps I will next add a few units to attempt to improve gameplay.