In my latest game I set out to get the Celtic achievement for building their 33rd city but it ended up being a great test of an ICS strategy as well.
I rigged the map a bit for the purposes of the achievement doing 2 civs on a standard size map (meant to do no city states, but ended up with 4). I also disabled barbs and used a forest map to take advantage of the Celt's bonus faith, but left the difficulty on Immortal (my usual setting).
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I played a few different starts because I ended up not being terribly happy with getting the free worker first (takes too long to get to the free and cheap settlers). I also played around with Stonehenge and/or Shrine early so that I could enhance faster, but at least as the Celts with forests nearby this didn't seem very helpful.
What I ended up doing was something like Monument -> Granary -> Pyramids -> Settler spam while managing to squeeze 2 scouts in there as well while waiting for research/policies to finish up. For policies, I went straight for the free settler and then started pumping them out of my capital. Granary can be a bit situational (I had 2 deer in my start radius) but even just 2 extra food can help speed the settler pump. Pyramids are a steal at 185 hammers. The 2 workers they provide are worth 140 hammers by themselves, so you are basically paying 45 hammers for a 25% worker speed boost, +1 culture, and +1 great engineer point.
One worker on road duty was pretty much able to keep up with the rate I was pumping out settlers. With proper placement you can connect 2 cities with 4 tiles of road, so that's only 2 road segments per city. The other worker (plus the once I got soon after from liberty) started improving any luxuries and then a few tiles around each city.
Early on happiness is mostly covered by focusing on building near luxuries first. Being unhappy (as long as you are above -10) isn't an issue at all since the capital isn't growing while pumping settlers and the other cities just make things worse if they grow too much before building some local happiness. Once I ran out of luxuries to settle Ceremonial Burial and +1 happy per trade route helped to keep the expansion going for a while. Eventually I had to take a short break from settler spam to build a colosseum in my capitol and pump out an extra crossbow to help defend against Japan's imminent attack (both my scouts were promoted to archers from huts and I got a chariot archer randomly from a CS). I did hit -10 unhappy for a couple turns at one point, but other than that I was able to keep up a pretty consistent settler spam from my capital the entire game so far.
City build order was almost always Shrine -> Colosseum -> Market -> Library -> University. After that I would build some food buildings if the city was having trouble filling the scientist slots and then as the Celts I pumped out the culture buildings to get Ceilidh Halls (Opera Houses) for the +3 happiness. You can basically let these cities grow as much as you want as long as you have enough local happiness to cover the population. Pagoda + Colosseum is 4 local happiness right there. If there are horses or ivory nearby you can grow to size 6. Theaters and/or UBs with happiness let you grow even larger. Each extra pop provides +1 gold from the trade route plus anything from the tiles it works, so paying for those happiness buildings isn't an issue. Science and culture building are also great to get.
I used the free great person from Liberty to rush Machu Pichu in a size 1 city I had built near a mountain. Trade route income is nearly half my my income, so this is at least a 10% total income boost. I took Messenger of the Gods for my Pantheon and then Ceremonial Burial and Pagodas when I founded my religion. When I enhanced I took Mosques and Itinerant Preachers. Mosques was a mistake--I was able to catch up and then pretty much buy Pagodas as fast as I could build Settlers in my capital (every 3-4 turns), but there's no way I'll ever have extra for a second building in each city. If I was more space limited this might have been a decent pick, but happiness from Shrines or Temples or Food from both would have been better picks in this game.
Itinerant Preachers is definitely *way* better than Religious Texts for an ICS empire (and probably in general). Around any given city you can fit 6 cities in the first ring (4 tiles away), 12 cities in the second ring (8 tiles away) and 24 cities in the third ring (12 tiles away). With Religious Texts you ca be pressured by at most 18 cities in the first 2 rings for a total of 180 pressure (144 before Printing Press). With Itinerant Preachers that third ring kicks in for a maximum of 42 cities giving 252 pressure. (As the Byzantines if you take both you could get as much as +450 pressure in a Holy city surrounded by 3 complete rings). Once you get going with this ICS approach, new cities will often convert on the very next turn.
Once you hit the Industrial Era you can pickup the Order opener for another +1 happy per city which, combined with the Liberty trade route bonus and Ceremonial Burial will completely negate the per-city unhappiness (all 3 count as global happiness). So, you can have absolutely as many cities as you can find land for and you can grow those cities as big as you can support through local happiness buildings even without any happiness from luxuries, natural wonders, etc.
Currently I'm at turn 210 and have just hit the Industrial Era (standard speed, immortal). I'm earning nearly 600 beakers per turn and I don't even have a library in my capital! I may never build a national college. I get more science from building another settler to found another city than I would get from building those things in my capital.
Liberty and Order are pretty essential to optimize the ICS strategy, but between them you have some options. I got 2 policies between finishing Liberty and hitting the Industrial Era. I sunk them into Rantionalism for the 15% science boost and +2 science per specialist. Piety would be good as well, though, for faster Shrine/Temple building and +1 faith from each (particularly as another civ that might need to focus faith buildings a bit more). Honor isn't bad either--Discipline should help defend against those AI attacks and Military Caste/Professional Army are great sources of free local happiness to let those cities grow. Even Tradition isn't terrible--if you start with it you can probably get the settlers going nearly as fast (you need an extra policy, but you get basically double culture at the start). Aristocracy could then help cover some extra happiness later, though the wonder production bonus probably won't be used much. Oligarchy can also be a good defensive policy--with cities packed so close, they can be a decent source of damage and can hit every tile in your empire. Patronage and Commerce seem like the least useful options to me since gold isn't really an issue once things get rolling, but I could see them being beneficial in certain situations.
Overall, this ended up being a very fun way to play. Science is through the roof and after founding my 33rd city on turn 210 and hitting the Industrial Era I could definitely win this game any way besides cultural. I'm looking forward to trying this out with some other civs on with more AIs (I'm thinking maybe Mayans on a Great Plains map with 4 civs total on a standard size map or 8 on a huge map).
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