When your settlement moves from a village (level 1) to level 2 you can choose its specialization -- Town, Fortress, Conclave.
That choice determines what types of improvements your city has available to it.
A town's improvements focus around getting you zee money and making zee food for itself and your other cities.
Fortresses are geared around producing really good soldiers and being really hard to conquer.
Conclaves are all about maximizing research and magic.
All cities can still build units, improve their production, and do some research. But it's not like in Beta 3 where every city you just build the same stuff. First, there's a lot more improvements to choose from. Second, improvements take the place of existing ones.
Cosmetically, in a screenshot, they look similar to what you had in Beta 3. But from a game mechanic point of view, they end up much smaller and you can quickly tell that THIS is my mega money making city and THIS city is my super soldier making city.
The beta 4 arguments about cities will involve saying that one type of city if over powered (or under powered).
I should just write the complaint posts myself to get it started:
"Conclaves are ridiculous, they make it too easy to win by throwing crazy spells around"
"Nonsense! Fortresses need to be nerved. While you're going up that tech tree and trying to get more essence, my unkillable knights will be wiping you out."
"You guys are idiots. While the egg-head magicians are trying to cast a better fireball and the grunts are doing acrobatic training, I'll have bought off all the elite mercenaries and made alliances with everyone who will crush you while I'm sipping on tea."