If you're playing an Empire faction, you can build a lot of cities. As you get more cities, it becomes more important to start them building them Workshop chain, as Slave Pens will nullify the Unrest penalty for Production (thereby circumventing part of the city count Unrest penalty). Also, with Death Magic, you can have Oppression on the city until they complete the Slave Pens. Once you have Slave Pens built, you don't need to reduce Unrest until you want that city to produce Research.
When expanding, keep in mind that if you get your additional cities' borders to connect to your first city's borders, they lose an Unrest penalty (I think it's 15% for not being connected). If you don't have the option for manual building placement enabled already, you might want to enable that to accelerate this process (it also helps to avoid releasing nearby monsters from their lairs).
As for what to start out with, I usually grow my first city to a Fortress (if it has at 2+ essence) before making any Pioneers. Cities won't down-level when the population drops. Fortresses have a bigger production boost than the other city types, and they can build better units. The essences are important because the city enchants you can play on the city to improve units produce increase the status by 1 per esssence. So, if you put the fire enchant on a city with 1 essence, they only get 1 bonus attack, but if the city has 3 essences, you get 3 bonus attack (and in the case of the bonus attack, that bonus is multiplied by the number of people that make up the unit, which is 3 to start, so you're looking at a difference of 3 vs 9 attack total right there, and it's fire damage which isn't reduced by armor). Air and Earth have the other good city enchants for training units.
I play on Insane difficulty with dense everything, so having a strong military early on is vital. I also play Empire, and I make a lot of cities.
