Initial money and loan amount are increased for high difficulties.
These special levels are made for fun and experimenting.
Difficulty settings are available from the Main Menu. Main Menu > Options > Mods Settings > Difficulty Tuning Mod
Changing difficulty options during a game is possible, but cost values displayed in the info panels will not update until reloading the game. Better to exit to the Main Menu, change the difficulty options, and load the game again.
Below is the list of the customizable parameters. When you change a difficulty parameter, the game difficulty is automatically set to "Custom" level. The custom values are being saved. Whenever you switch from a named difficulty to the custom level, all your previously set custom values are restored.
Construction and upkeep cost
Separate for service buildings, public transportation, roads and for everything else (park, plazas, pathways, trees, and monuments).
Strictly speaking, the construction cost is not critical. Increasing it even in 100 times will not make the game unplayable. You will just have to wait a very long time before constructing something. The upkeep is what is important.
In general, at higher difficulties, roads are set to be the most expensive - you should plan your traffic ways and road types more thoroughly. On the other hand, upkeep for public transportation is not changed much. You do not have to restrain yourself from building a good public transport network.
Demand offset and multiplier.
Calculated by the following formula: New_Demand=(Original_Demand+Offset)*Multiplier
In very general meaning, changing the offset will result in changing the city growth condition (grow or do not grow). Changing the multiplier will result in changing the city growth speed (fast grow or slow grow).
Ground and noise pollution radius
Bigger radius for high difficulties. Can be set from 0 (no pollution) to x5 in the custom options. Only power, water, and garbage facilities are affected.
Relocation cost
Varies from 0 (free relocation) to 80% of the construction cost depending on difficulty. In vanilla you can relocate a building at 20% of its cost.
Maximum slope for roads, railroads, and metro
Easy: 30%, normal: 25%, expert: 19%, impossible: 15%. Can be set from 5% (very gentle) to 100% (45 deg.) in the custom options. Please note that setting the slope limit below 15% will make road/track constructing tricky (especially tunnel entrances). Using Fine Road Heights mod may help.
Area cost multiplier
Expanding is quite expensive at higher difficulties.
Reward percentage
Amount of money you get when a milestone is reached. No reward at higher difficulties.
Start money amount
Depending on map, sometimes it is impossible to construct the minimum required facilities, when you start a new game with the default amount of money at one of the highest difficulties. Therefore the start money is increased a little bit for the Challenge and Impossible difficulties. Can be set from 0 to ₡500.000 in the custom options.
Loans
Loan amount increased for higher difficulties. Can be set from x0.25 to x10 in the custom options. Loan length changes by formula (1+x)/2, where x is the loan amount multiplier.
About the formula. If increase the loan amount without changing the loan length, the payment per week would be too high. On the other hand, if increase the loan length at the same rate as the loan amount, the repay period would be too long. So I decided to implement the medium way. For example, if increase the loan amount in 3 times, the loan length will be increased in (3+1)/2=2 times.
Population required to unlock milestones
The higher difficulty the later the game stuff is unlocked. You can set it to 0 to unlock everything from the beginning.
Land value requirements for level up of Residential and Commercial buildings
High requirements at higher difficulties. Education and wealth requirements are not changed.
Service score requirements for level up of Industrial and Office buildings
High requirements at higher difficulties. Education requirements are not changed.