Some important news if you are trying to play in Cities: Skylines in offline mode with Workshop items. A new Steam beta client version has been released which you can now test which allows workshop items in offline mode. See the note below from Paradox on how to get this working.
I do not know how long Steam runs the beta cycle before they will put their patch in the public release of the client so if you want to try it out already, confirm it also works for you (worked for me), please feel free to do so.
To join the Steam client beta, go to your Steam settings, in the Account section (which should be selected by default) there is a Beta participation area, click Change and in the list select Steam Beta Update. Steam will probably require you to restart it in order to install the beta version, do what it says.
Once installed, if you wish to use the offline mode, toggle it like you would normally do, launch the game normally and your workshop items which are already subscribed and installed should remain in the Content Manager just as it currently does when you are online.
PS: Strangely, I had a problem when launching the game after the update and I would get: “Failed to Contact Key Server”, it is unlikely to happen for everybody, but if it does for you, I just followed the steps from the Steam support (https://support.steampowered.com/kb_…2204-OPGC-4862) to get it sorted out.
Problem Occurs After Updating Steam
If this problem occurred directly after a Steam update and the firewall is already disabled, please try this process:
1. Log out and exit Steam
2. Navigate to C:\Program Files\Steam (the default location). If you have installed Steam to a different folder, please locate that folder now.
3. Rename the file “ClientRegistry.blob” to ClientRegistryOld.blob and then close the Steam folder
4. Launch Steam and login. Steam should update, re-create the file, and then you will be able to play