I say this because one aspect of playing in Fenrecz was that it was really difficult to get the players out of the city -- in fact, I never managed it all, despite numerous hooks that led outside of the city walls. Now, it should be mentioned that this isn't necessarily bad in that if a party really wants to stay entirely within a city the entire time, they should be allowed to in the sense that a true sandbox game goes where player decisions take it and no further. On the other hand, it does lead to one major difficulty, which was that the players were stuck between the city being familiar and alien.
Since the campaign had only ever taken place inside this giant city, it felt unnatural for the players to be newcomers to it, even if that were most of their character backgrounds. This was exacerbated by one of the players asking for a background that would have placed him in the city previously -- so we had a theoretical knowledge imbalance and a practical lack of imbalance that felt like it shouldn't be even as it made logical sense.
Now, I'm sure that a more experienced or more skilled DM could have dealt with this in an elegant fashion. With our group, it wound up making sessions awkward as the players weren't sure to what degree they should be familiar with the city -- the solution, it seems to me, is to start a campaign off in a hamlet or such, a smaller settlement outside of the core cities of the surrounding area. This way, the players can still have a game that centers around a big city -- they just have to make the decision to travel there first. And, in theory, a party would be just as likely to go for dungeon- or wilderness-based adventures, without the draw of a city with conceivably everything they could desire contained within (and, probably just as importantly, full of inns and other "safe" places that are always within easy reach).