This was a talk given at GDC 2012, the morning after the SimCity 2013 announcement, on the internals of a new simulation engine, called GlassBox. This was originally targeted at a potential series of mobile games, before we (Maxis) regained the SimCity franchise from EA (confusing, eh?), and hence redirected development towards that.
The talk covers the basic pieces that make up the simulation architecture, the things we considered in putting it together, and explains why we took this approach.
Slides in PDF format (4MB)
Videos from the talk:
Dan Moskowitz (who showed off various SimCity examples after the slides) has more here.