Simon Fox wrote on the SimCity Blog that responding to fan calls for a mode that could be played offline and saved and reloaded required a great deal of effort.
"[O]ur entire architecture was written to support [networked play], from the way that the simulation works to the way that you communicate across a region of cities," Fox said. "So yes, while someone was able to remove the 'time check' shortly after launch, they were unable to perform key actions like communicating with other cities that they had created locally, or with the rest of their region(s), or even saving the current state of their cities."
Fox went on to detail the changes necessary to get an offline mode up and running. For instance, the original game relied on communicating with servers for region status and other information via a Java-based system. Letting this information be distributed offline meant rewriting the entire system in C++. Optimizations were necessary to ensure all computers that could already run SimCity in online mode could handle these additional calculations.
"Even things that seem trivial, like the way that cities are saved and loaded, had to be completely reworked in order to make this feature function correctly," Fox added.
The offline mode is in the final stages of testing and will arrive with SimCity's Update 10. The update will also allow offline players to install mods, though they appear to to be limited to cosmetic customizations.