Librande: First of all, when we started the project, and there were just a few people on the team, we all agreed that we didn't want this game to be called SimCity 5. We just wanted to call it SimCity, because if we had a 5 on the box, everybody would think it had to be SimCity 4 with more stuff thrown in. That had the potential to be quite alienating, because SimCity 4 was already too complicated for a lot of people. That was the feedback we had gotten.

Once we made that title decision, it was very liberating -- we felt like, "OK, now we can reimagine what the brand might be and how cities are built, almost from scratch."

Technically, the big difference is the "GlassBox" engine that we have, in which all the agents promote a bottom-up simulation. All the previous SimCity games were literally built on spreadsheets where you would type a number into a grid cell, and then it propagated out into adjacent grid cells, and the whole city was a formula.

SimCity 4 was literally prototyped in Excel. There were no graphics -- it was just a bunch of numbers -- but you could type a code that represented a particular type of building and the formulae built into the spreadsheet would then decide how much power it had and how many people would work there. It just statically calculated the city as if it were a bunch of snapshots.

A fire breaks out in the city designed by We Are The Champignons.

Because our SimCity -- the new SimCity -- is really about getting these agents to move around, it's much more about flows. Things have to be in motion. I can't look at anybody's city as a screenshot and tell you what's going on; I have to see it live and moving before I can fully understand if your roads are OK, if your power is flowing, if your water is flowing, if your sewage is getting dumped out, if your garbage is getting picked up, and so on. All that stuff depends on trucks actually getting to the garbage cans, for example, and there's no way to tell that through a snapshot.

Sims queue for the bus at dawn.

Once we made that decision -- to go with an agent-driven simulation and make it work from the bottom up -- then all the design has to work around that. The largest part of the design work was to say: "Now that we know agents are going to run this, how do schools work with those agents? How do fire and police systems work with these agents? How do time systems work?" All the previous editions of SimCity never had to deal with that question -- they could just make a little table of crimes per capita and run those equations.

Manaugh: When you turned things over to the agents, did that have any kind of spatial effect on game play that you weren't expecting?

Librande: It had an effect, but it was one that we were expecting. Because everything has to be in motion, we had to have good calculations about how distance and time are tied together. We had to do a lot of measurements about how long it would really take for one guy to walk from one side of the city to the other, in real time, and then what that should be in game time -- including how fast the cars needed to move in relationship to the people walking in order to make it look right, compared to how fast would they really be moving, both in game time and real time. We had all these issues where the cars would be moving at eighty miles an hour in real time, but they looked really slow in the game, or where the people were walking way, way too fast, but actually they were only walking at two miles an hour.