Optional services
Education
Everyone can live without schools. No one has to be educated to be able to work. But schools can greatly increase the amount of profits, as they will increase the level of all buildings, and thus tax income.
Elementary school has small reach, so it should be build on each estate, sometimes even more often. High school has larger reach and capacity, and university educates great amounts of people and has reach on almost whole city.
The higher building level, the more educated people must work/live in it.
Recreation
Parks, playgrounds and squares improve ground value around them and provides recreation to citizens. Their presence is required for high level buildings of all categories.
Transportation
Interestingly, city communication is considered as service as well and it helps in increasing building level.
Buses and metro will greatly help your roads with traffic jams, which will allow more fluent move on city exits, so that trucks will have easier road.
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