Basics:

There seems to be a lot of confusion on how best to proceed with building your empire and competing with AI from a builder point of view. I have mostly been playing Huge maps exclusively, and I try to compete with AI with armies and specialized cities instead of just beefing up my Sovereign with Beastmaster and magery. So I will try to explain how to get your empire up as soon as possible to fight with them on equal grounds.

So I'll lay a bit of groundwork first. What is the one thing that decides the power of your cities? Research! Get that research up as soon as possible and all the options for growing powerful cities opens up. With lots research you can open the production line of buildings,(Workshop, Mason, Mill), the food line (Garden, Granary, Storehouse) and of course, more research! (Study, School, College). So from Research you get options for Food, Production, Gildar and more Research. That means the primary goal for your early game should be getting that Research up as soon as possible! The AI's reason to exist is to slow down your simcity power, so only get army (Warfare) and (Magic) techs if you feel they are dangerous. If they are not you can safely keep teching Civilization paths as that will just increase your Simcity power and get you ahead of the AI.

Growing Big Cities:

With that out of the way, we get to the most important and core problem for simcity. Growth. (Growth makes your cities level up faster) So how do we get growth? You have 3 possible ways: 

- The basic one: Food. Lots of food gives 3 Growth, medium food gives 2 growth, low food gives 1 growth.  

Low Food is where your progress is slowing. (if your cities are big, they will have low food always, so expect your good cities to have low food if you are pushing your game properly)

- Sovereign's Call (Life1 spell): The easy one, Click and get +2 Growth. This is why many people think Kingdom is good in the beginning. 

- Consulates: Get those Outposts going, each Consulate (Cooperation Tech) gives +1 Growth. This is production intensive, so a good balance between production,research and food is needed to make these work. These are the buildings you need as empire to catch up to the Kingdoms as Empire.  

Food is what limits your city leveling, but that is solved by the Food Line of Improvements, unlocked by more Research of course!  (Gardens, Granaries, Storehouses), and Towns with the Grocer, Bakery etc. faction food increase stuff later. When you want Towns.. ^^

So your basic 3-4 grain Villages will grow up with just Research and no Town support. For late game the Sorcery Tech adds Nature's Growth  will make your pioneer spam conclaves able to "grow up" later the game. The Gentle Rain (Aeromancy Mage Ability) spell will greatly increase your city food cap per Essence for just the cost of one Essence Slot. So if you have a mage Champion or Sovereign, this spell will greatly help your Simcity efforts. If you do not have either of these spells, it will be hard to get your cities up properly, but getting at least one of them, will make you able to reach the precious lvl5 on your high grain cities. If you can attain both spells, you are sure to get that lvl 5 city location with 2+ essences, so your capital will be assured to get level 5.

So what do the types of cities specialize in?

So now we have a fair graps of the basics hopefully. Lvl 2 cities gives more Research/Production/Gildar so getting them up as soon as possible should always be your main priority. This is why Life Magic is more powerful in the beginning (easy +2 Growth at the cost of one Essence Slot). Does that mean you have to be Kingdom to succeed? No, it just means more work if you are Empire. (Empire gets more powerful later, and more fun ^^ )

So what are the advantages for the City Upgrades? 

- Conclaves gives more Research

- Fortresses gives more Production

- Towns gives more Gildar (MONEY $$!) 

How to level your cities as soon as possible?  

So now we get to the real guide part, you know the basics and what to do to get perfect Simcity, so you can make your own strategy:

My strat is that if the first town has lots of essences, I make it a fortress. (that means more production at start, and good troops to defend against AI) If it has bad essence and good production I would consider making it a conclave instead.

Never make your capital a town. Towns are for the late game (they get really awesome then). You need good Production and Research at the beginning and that means choosing Fortresses and Conclaves, not Towns. Both a Fortress and Conclave start is viable options, so decide what to make your capital depending of your starting essence and materials. I would make anything with 2+ Essence and 3+ Materials a Fortress, and if you get a "bad" start with only 1 essence and 3-4 Materials, I would make it a Conclave instead. 

Villages founded after capital up to 4-5 or so should always be conclaves. Conclaves boost research and as such boosts growth and production. If you find a 3 essence spot and haven't made a fortress yet, conclave is still good, but you might need a place to make troops from, so you can never go wrong with a 3-essence Fortress location either. Conclaves gets more Research from more Essences via the Alchemy improvement line. (+Mana, then Research, and Finally Materials per essence) So Oracle improvement at lvl 3, is your goto improvement, if you are unsure what to pick.The exception is picking one Conclave with high essence for Archivist (Specialized research city, so get University etc. there for the best Research city possible) and one Conclave for Scroll Scribe. The Scroll Scribe will sell you Blizzard Scrolls, and those will make you able to beat a lot of high level AI's, Every Conclave except for those should always select Oracle to max Research and Production. You can also consider skipping the Scroll Scribe unless you play higher level of AI. The Scroll Scribe is mostly important for beating high AI levels.

So I just make all my towns conclaves from now on? Well that can work, but it's not optimal. If a location has for instance 1 food and high production, Conclave is a good option no matter your calculations. But don't expect it to get it to lvl 3 until late game. Make it a pure conclave research site, getting the conclave research buildings up as soon as possible and a pioneer spammer. Pioneer spammer towns are the ones you want to boost your other cities. Remember the Consulate building? Yes, you need Pioneers to help the other towns grow big. So getting a few specialized towns spamming pioneers will help your overall Simcity power.

So look for at least 1-2 sites with 1 food - 5 Production, and make them Conclave Pioneer spammers. 2 food-4 production sites etc, works as well if you can't find sites with these values, but it might be harder to produce as many pioneers as they can and you might be tempted to let them upgrade to lvl 3 instead, and grow powerful

If you find at least 2 or 3 sites like these, they will give you the research you need to compete with AI's at the beginning, as well as outposts for your "main sites". These are very important. Since they can't grow to level 3, make them spam pioneers for your other towns so they can grow instead! (Make Outposts, then Consulates) An added bonus at higher levels is the possibility to make Armories etc. for attack and defense bonuses to defend against AI invasions.

So what about towns? Are they useless? No, they are awesome, but only later in the game. If you already have a 3 essence Fortress site, look for a 3 Town Essence site, and if you can't find one a 2 essence high production site will work just as well (hopefully, just get it going somewhat soon). What you want for that perfect town site is as huge Material Count as possible. That means Clay pits, Arcane Forge(Spell)/Enchanted Hammers(Earth1 Spell)  for more materials, and that is waht you are looking for a perfect gildar factory town. The lvl 5 town improvement gives you +5 Gildar per material, so make that town picking the +% Gildar improvements as well as max Materials, and you can fund whatever end game army you field. But only do that with 1 town, unless lots of Gildar turns you on. 

So now you are done with what to do with the early towns. What about the rest?  Well that is easy:

If you need more Research to keep up with AI: Make them conclaves

If you need more Gildar to fund your army: Make them towns. (added bonus of giving your army +5% HP, so you need some towns later in the game)

If you need production to produce more troops to fight!; Fortresses.  (I prefer quality over quantity, but if you prefer the latter, the more Fortresses the merrier) 

Final Note: Only for Large maps+ (or lots of cities empires)

You might want some more Fortresses to compensate for the higher amount of towns. Prison the lvl 4 Fortress upgrade (-10% unrest for all cities) and Onyx Throne  the lvl 5 fortress upgrade (-30% unrest for all cities) makes it possible to control a huge empire (10+ cities), so you need to implement at least 2-3 Fortresses with Prison, or one Fortress with Onyx Throne and Prison, to make it viable. I recommend making a Fortress with your 6th or 7th Village, if you want to keep your 1st Fortress Great Arena and quality troops spammer, (+1 Attack +3 HP for trained Units) For large maps, you might want to arenas, but usually for smaller maps you only need one of those.