Now I want to play emperor, but the AI are getting to the Classical Era before I even finish two techs, what's the best approach with the Emperor tier difficulty?
Playing better than on king.
You could try a Horseman rush, followed by ICS.
Horseman rush goes like this:
1. research Animal Husbandry to reveal horses.
2. build Scout, Worker, Settler
3. research Wheel, then Horseback riding
4. If capital doesn't have horses when the Settler is finished settle your 2nd city directly on a nearby Horse resource (to save the time you would normally need to build a Pasture). If capital has horses just make the city in a good spot. Preferably near a Horse resource.
5. Build 2 Horsemen in capital, 1 Horseman in 2nd city.
6. Two possibilites:
a) You're doing a 4 Horse rush. Buy a 4th horseman when the 3rd is 1 turn away from being finished. This costs 410 gold so save it. Then you use the Horsemen to attack your nearest neighbour.
b) You're doing a 2+2 Horse rush. 1 turn befor you finish first Horseman in capital buy a Horseman (in the capital). You attack your nearest neighbour with them and then when your other 2 horsemen are built you send them as reinforcements.
After researching HBR you go for whatever tech you need for your nearby luxuries, then you research Masonry (for Coliseums) and Writing (for Libraries). Possibly you can research Writing first and Masonry second. That depends on your situation and your civilization. For example China's UB is a Library that gives gold.
ICS is basically about spamming cities, but you need a few things first. Most importantly you need to be able to build Coliseums and Libraries. You'll also need a maritime city state ally, but that's not that urgent.
The trick with ICS is simple. A city produces 2 unhapiness + 1 per population. A Coliseum gives 4 happiness for 3 gold upkeep. A size 2 city with a coliseum is neutral in regard to hapiness and gives 3.5 trade gold income. So basically it covers it's own cost. But it also produces science, especially when you also have a Library. If you run 1 scientist specialist in such a city (requires Library) you'll be making 6 science per turn from it. And if the other citizen is working a tile with a Trading Post the city is also providing you with 1.5 gold per turn. And it costs no hapiness because you have a coliseum so you can have as many of these cities as you want.
So basically what you do is you keep spamming these cities and settle them as closely together as possible. then when they grow to size 2 you click "avoid growth" to keep them that way.
One thing that helps a lot is taking Liberty -> Citizenship -> Meritocracy as your social policies. Liberty will speed up settler production and Meritocracy givs +1 hapiness per city in your trade network. So if you wait until you get Meritocracy before starting ICS you can grow your filler cities to size 3 which basically means either +2 gold per city (if the extra citizen is working a Trading Post) or +3 Science per city (if the extra citizen is working as a scientist specialist).
You'll also want to bild the Forbidden Palace wonder. It's the best wonder in the game if you're using ICS. It reduces unhapiness per city by 1 so you can grow cities to size 4 which is another 2 gold per city. That doesn't sound like much, but when you have 40 cities it makes a difference.
For civilization I'd suggest one of these:
Greece - best Horseman rush in the game, cheap city state alliances
Mongols - almost as good as Greeks at Horseman rush and better if you keep warmongering into the Medieval Era
China - unique building is great for ICS, you will be swimming in cash
Egypt - unique building is good for ICS, +2 happiness and culture for no upkeep. Allows you to grow filler cities 2 sizes larger than normal.
France - unique ability is great for ICS, you get your early policies quicker and you may actually get a 4th policy naturally (go for Freedom)
Babylon - you get great scientists 2x as fast so you tech faster
Edit: I noticed I forgot to mention maritime food. So basically if you're the ally of a maritime city state you get 2 food in every city before Renaissance Era and 3 food in every city in Renaissance and later eras. If you get two of these a city that is literally anywhere can grow to size 4 just from the basic 2 food provided by the city tile and the 6 food from both Maritime allies. So you can put your filler city in the middle of the desert or even on snow and it will still grow and provide you it's benefits.
You will also need production cities. That's your capital, the captials of the enemies you conquer, and maybe your 2nd and 3rd city you settle. You use these cities to build units and wonders.
Also check
this post from
this thread. It's got pictures how to use Horsemen properly.
After you have Horseback Riding, Writing, and Masonry the key technologies are Banking (for Forbidden Palace) and Rifling. Once you have rifling go for Dynamite. Burn Great Scientist to get to these last two techs quciker, they are gamechanging. If you're the first to get Rifles and Artillery your army will be unstoppable.
Apart from Coliseum and Library your filler cities will only build building that have 0 upkeep (stuff like Markets or the Egyptian Burial Tomb) and possibly Universities. Universities depend on whether you want more science or more gold.