It's worth keeping in mind that this is just a suggested build. Part of the attraction of Illyriad is that it allows for many different playstyles and builds.

That said, this guide assumes that you have a basic aim when building a city (in general for this particular guide) to have approx. 26,000 population with supporting 60-80% taxes supporting 25,000 T2 cav or equivalent. This is usually sufficient to ensure that you can reach 10 cities without too much trouble, and can push to cities 11+ with some adjustments. The changes required to reach cities 10+ are different enough to normal build patterns, and so far down the development of an account, that they are not covered by this guide.

Required in every city

• Barracks: Needed for troop production.
  
• Consulate: Needed for diplomatic unit production, and sending diplomatic units on missions.
  
• Mage Tower: Needed for mana generation.
  
• Library: Needed for research.
  
• Warehouse: Needed for storage.
  
• Marketplace: Needed for trade.
  
• Flourmill: Needed for food boost.
  
• Saddlemaker: Needed for gold/saddles.

Specialization
After that the decision on what to build, or not build, is dependent on the city. To achieve the greatest returns it is usually best to specialize your cities. One city produces one unit type (like infantry, cavalry, etc). The only exception is if you are building a diplomatic unit city. It is often common for a city like that to build one diplo unit type and one military unit type.

Archers

• Brewery (if Elf)
  
• Fletcher
  
• Tannery
  
• Archers Field x3
  
• Bowyer OR Leather Armourer (you may want two of these)
  
  Spearmen
  
  • Brewery (if Orc)
    
  • Spearmaker
    
  • Tannery (if building T1 spearmen)
    
  • Blacksmith
    
  • Forge
    
  • Spearman's Billets x3
    
  • Spearsmith OR Leather Armourer OR Plate Armourer OR Chain Armourer (x2?)
  
  Infantry
  
  • Brewery (if Dwarf)
    
  • Blacksmith
    
  • Forge
    
  • Infantry Quarters x3
    
  • Weaponsmith OR Plate Armourer OR Chain Armouer (x2?)
  
  Cavalry
  
  • Brewery (if Human)
    
  • Tannery (if building T1 cavalry)
    
  • Blacksmith
    
  • Spearmaker
    
  • Forge
    
  • Cavalry Parade Ground x3
    
  • Spearsmith OR Plate Armourer OR Chain Armourer OR Horse Trainer OR Leather Armourer (x2?)
  
  It is important with the following two city builds to be mindful of your basic resource rate per hour. It can be very tight to fit in 4 different unit maintenance reduction buildings, but it is possible with some forethought.
  
  Diplomatic unit city
  
  • Bookbinder
    
  • Foreign Office (dependent)
    
  • Diplo maint reduction building x3
    
  • Spearman's billets x1 or Archers Fields x1
  
  Geo or Magic city
  
  • Geomancers Retreat x3
    
  • Spearman's billets x1 or Archers Fields x1
  
  Remaining slots
  
  • Resource buildings like Carpentry etc.
    
  • Temporary storehouse for lv20 Wall.
    
  • Book binders. Watch research/hour.
    
  • Skinners Guild. Crafting/selling.
    
  • Cotters. Watch pop.
    
  • One trade city: Trade Office and Merchant's Guild. Inf/Spr city with 5 wood.