This is a "rule of the thumb" list of which city-improvements are needed and what to do to keep your cities happy to get size 20 cities by smooth celebration:
To celebrate your cities need happiness. You can get happiness in a city by different paths. Some are more efficient than others.
1) Increace luxury-rate at the cost of less tax and/or science
2) Build cityimprovements that give happiness: Temples, Colosseum, etc
3) Build happiness Wonders
4) Give your city a little more trade by micromanagement
5) Boost trade in a city by making traderoutes
6) Increace the happiness coming from from trade and luxury by building marktetplace, banks and stock-exchange
The noob strategy: 1) + 2) + 3)
The first 1) 2) 3) are what most new players use to increace happiness. But its a costly and time-consuming strategy. Increaced luxury-rate will make your science and economy suffer. Colosseum and Cathedrals are expensive to build, and have high maintainence cost (gold) pr turn, making your economy suffer even more. Happiness wonders are better. This way you save a lot of gold for building/buying city-improvements, and cost pr turn for upkeep. But Wonders require a lot of production (shields) to get, and are extremely expensive to buy. And there is a big probability you dont get this wonder in time, when you really need that happiness..
The less noob strategy: 2) + 3)
The only exception to the strategy described above are temples. They are cheap to build and buy, and will make 2 citizens happy when combined with mystesism. Oracle will give 1 extra citizen happy, adding up to 3 citizens happy from temples.
The Trade strategy: 4) + 5) +6)
Trade can be converted into gold, bulbs or happiness. Cities high on trade will be easier to keep happy. Sea-tiles give lots of trade. Cities with harbours will get both food and trade from sea-tiles. The Luxury-/Tax-/Science-rate will determine how much trade till be used to produce luxury, gold or bulbs. Marketplaces will increace gold and luxury by 50%. Banks will also increace gold and luxury by 50%. This imply that marketplaces and banks will boost happiness defined by city-trade and luxury-rate you set. Marketplaces and banks will also give an additional boost your economy by 50% each. And marktesplaces have no cost pr turn! When you start adding traderoutes that add trade to each city, marketplaces and banks totally outperform Colosseums, Cathedrals and happiness Wonders..
Micromanagement is powerful early in game. Later in game when game is getting more complex, efficient use of governors can be extremely powerful. Use custom made govenors to manage your celebrating cities. This will save you time. Time that can be used to chat with your alliance and conspire to beat pro players
City-improvements needed to celebrate:
A city need to be at size 3 to be able to celebrate. Use the "Granary-trick" to get size 3 cities faster.
I consider traderoutes as a city-improvement. 4 traderoutes pr city is more important than banks. You should fill up your cities with traderouts as early and as fast as posstble. Sell aquaducts to get them faster. Buy caravans to keep them flowing. Some players have a fetish on building expensive city-buildings before caravans. Stop making cityimprovements when your cities need trade to become happy. Traderoutes will make your cities happy. Traderoutes are city-improvements!
As we see there is 3 strategies to celebration. Use them all wisely. Here is a script I use to make a smooth celebration:
Size 3 to 8:
With 30% lux, your cities will usually start rapture if you micromanage them and make sure they get enough trade. But from size 5 to size 8 the cities need temples + mystesism to have a smooth celebration.
A city built on grassland and plain usually need only a temple. Temple + Mystesism will make 2 citizens happy.
Cities built on hills, mountains or forest usually also need marketplaces to celebrate form size 6 to size 8.
Harbours that enable you to use sea-tiles will add happiness to your cities because sea-tiles have high trade. But remember, if you have irrigated some plains and grassland, harbours are not needed. Build only harbours in cities that have too little food for some reason.
During this phase its wise to get at least 1 or 2 internal traderoutes to boost happiness and economy and prepare for next push toward size 12.
Size 8 to 12:
A city built on grassland and plain usually need temple and marketplace. They also need traderoutes and harbours to get a smooth rapture.
I prefer to build marketplaces before harbours. Marketplaces boost gold coming form tax. Higher income will make it easier to finance both harbours and aquaducts.
Cities built on hills, mountains or forest usually also need banks and traderotues to have smooth rapture form size 8 to size 12.
During this phase its wise to get all the missing internal traderoutes to boost happiness and economy and prepare for next push toward size 20.
If you have an allie, start doing some international traderoutes.
Size 12 to size 20:
A city built on grassland and plain usually need temple, harbours, marketplace and bank. They also need traderoutes to rapture.
Cities built on hills, mountains or forest usually also need Stock-exchange, or in rare circumstances Coloseum, to celebrate form size 12 to size 20.
During this phase you must make the missing international traderoutes to boost happiness, economy amd science. With missing internal traderoutes it will become extremely expensive to reach size 20.
Remember you can sell auqaducts when you cities reach size 12 to finance banks and caravans. And you can sell aquaducts and sewers to buy caravans, frigates, libraries, unversities, stock-exchange when you have reached maximum size in your celebraing cities. You only need aquaducts & sewers to grow your cities. After that they arent useful for anything. Sell them!
And remember the granary-trick: Sel granaries too!!l
Hope this will help for you all to develop your LargePoxing skills