Below are the assemblies you might need when making your mod. There might be some that aren’t listed below but these are the ones for Cities: Skylines and Unity.
You can find these assemblies in your steamapps. (See folder structure)
Check out the reverse engineering page for more information how these can be usefull. You will also need to reference these when settup up your project to be able to use them.
The custom Colossal UI and a lot other framework classes which are used in the game. Things like: DataBinding, Globalization, HTTP, Importers, IO, Math, Packaging, Plugins, Security, Steamworks and Threading.
This contains almost all of the gameplay related classes. From the road system to citizens and from buildings to the chirper. You will find most things in this assembly.
The Unity game engine which the game was built in. Check out the scripting reference on unity for more information. Remember that this doesn’t contain UnityEditor only everything in the UnityEngine namespace.