Slums in Cities Skylines are useless. The only point of a "slum" in that game is to convert the slum into a richer neighborhood.
In contrast: creating Slums in Ceasar, Pharaoh, or Tropico is an explicit strategy. Why? Because low-income industries (such as mining or farming) cannot afford better housing. The slum is the best that you can give that population as a leader.
In Ceasar, they're extremely cheeky about the highest class of citizens. Only plebeians work. If you "upgrade" housing to the point of attracting patricians, you LOSE workers (because patricians don't work!!). There are entire strategies in the game about attracting as many plebeians (working-class) citizens needed to have a functioning economy, but then converting the excess workers into Patricians for higher-tax collection.
As such, you need to keep the plebeians purposefully at a lower class to keep your industrial sector working. If you convert everyone into patricians, no work would get done at all. Its a challenge to build road-networks that separate the housing units, but provide the workers needed to serve the upper class (ex: services such as bath-houses and marketplaces are run by plebeians, but should be placed inside of Patrician neighborhoods)
To be fair, almost everything in Cities felt useless. It didn't really feel like much of a game at all. Nothing you did really actually mattered at all. Cities was more like an interactive city model building toy than what I'd think of as a game.
Ceasar, Pharoh and Tropico are actual games with goals, win conditions and a point.
I didn't like Cities mostly because nothing you did mattered. Not just slums or anything else. You can kill off half the city by accidently routing waste dumping to the city's drinking water and within 15-20 minutes your city's back to being fully populated.
Trying to play Cities strategically the way you would those other games is going to be disappointing no matter how you build the city. It's just not that kind of game.
If I recall correctly, in at least some of the Caesar/Pharaoh/Zeus/Emperor games, access to workers is a binary "Did a worker walk by here recently?" check, and if the answer is yes, then the whole city's population of workers is available to the building. So to serve a remote cluster of mines or whatever, you can have a "slum" that's one or two houses big.
There might be seven or eight levels of "common" housing that provides workers and then two or three of "upper class" housing that doesn't. But at least to me, a block of insulae isn't a "slum" in the game--a couple of huts with no water or food are.
Sim City btw, does the same thing with Residential / Commercial / Industrial zones. All "traffic" start from a residential zone, then goes to a industrial zone, and finally ends in commercial.
That means a singular tile of "Residential" zone can "feed" the entire industrial sector. While a singular "Commercial" tile can serve as where the industry drops off their goods.
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So I knew that fact for Sim City (and took advantage of it in some designs). I never really knew that for Caesar / Pharaoh.
In contrast: creating Slums in Ceasar, Pharaoh, or Tropico is an explicit strategy. Why? Because low-income industries (such as mining or farming) cannot afford better housing. The slum is the best that you can give that population as a leader.
In Ceasar, they're extremely cheeky about the highest class of citizens. Only plebeians work. If you "upgrade" housing to the point of attracting patricians, you LOSE workers (because patricians don't work!!). There are entire strategies in the game about attracting as many plebeians (working-class) citizens needed to have a functioning economy, but then converting the excess workers into Patricians for higher-tax collection.
As such, you need to keep the plebeians purposefully at a lower class to keep your industrial sector working. If you convert everyone into patricians, no work would get done at all. Its a challenge to build road-networks that separate the housing units, but provide the workers needed to serve the upper class (ex: services such as bath-houses and marketplaces are run by plebeians, but should be placed inside of Patrician neighborhoods)