All your city is managed in your local PC. Everything runs fine, only saving is cloud. Proof is that people have played though server crashes without noticing,

All your city is managed in your local PC. Everything runs fine, only saving is cloud. Proof is that people have played though server crashes without noticing, up to 40 minutes. But, region management is server side. So basically, if one manages to skip log in and makes save-s local then one can play single city offline. This is as much information as I have at the moment.