If this is the case and you are using mods please try the following:
- Right click Cities in Steam
- Click "Properties"
- Click "Set Launch Options"
- Paste in: -noWorkshop -disableMods
What this does is disable all your mods and Steam workshop assets; if the game launches fine after this it means that one or more of your mods or custom assets are causing the problem. The solution is to remove your mods or turn them on one by one to see which mod / asset it causing the issue and then either remove it or update it.
There is a user-maintained page which documents known bad or outdated mods:
IF THIS DOESN'T WORK:
Find the output_log.txt:
Open the file (with notepad) and look at the top, if you see the following then the problem isn't actually Cities: Skylines but another program: Citrix Workspace.
The solution is to uninstall Citrix and replace it with the version in the Windows 10 Store, as that one lacks the feature that causes this problem to older Unity games.
If you get an error.log file from the game crash, check for anything like this at the top:
96% memory in use.
16317 MB physical memory [811 MB free].
17341 MB paging file [3 MB free].
The game has run out of available memory, and Windows has exhausted its swap space. Please:
- expand your Windows paging file maximum size to 32gb
- unsubscribe from any mods and assets you don't actually need or use - they all consume memory all the time, even when not in use in the current city
- exit every other app you can before starting the game.