I recall reading somewhere online that it was feasible to get from landmark to landmark in Assassin's Creed: Brotherhood by looking up Rome in Google Maps. This piqued my interest, are all the cities in the AC series accurate in their layout of the landmarks?
I don't know about Florence or Venice, but because Brotherhood mostly takes place in Rome, an actual Renaissance scholar was consulted during the design of the city of Rome. Read more here
The city itself was not meant to be extremely accurate as possible due to game constraints. Here is an excerpt from the article:
The famous Roman Colosseum still looms above virtual Rome, but its in-game shape is circular rather than elliptical. That's because the challenge of making different shadows for all sides of the building would have driven game designers nuts, Simonetta said.
"It's fine that you have a Colosseum that's not elliptical, because it looks like the Colosseum," Simonetta told LiveScience. "The fact that you can go inside these buildings [in the game] is incredible."
Rome in 1503 should look similar in architectural style to the city of Florence featured in "Assassin's Creed 2." But the art director for "Assassin's Creed: Brotherhood" decided to give Rome the Baroque architectural style of the late 16th century, so that gamers would have a new-looking virtual playground that is also more recognizable to modern eyes.
The first paragraph did allude to the fact that Florence and Venice were also recreated using similar methods.
I was just in Florence and Venice a few weeks ago and I was surprised that I recognized a few landmarks from the games (the Ponte Vecchio, where AC2 opens, actually took me by surprise when I was on it).
Looking at the in-game maps again now that I've spent time walking the two locations, I can say that the very rough locations of major landmarks are correct but the maps are significantly condensed versions of the cities. Neither Florence nor Venice are big places but they still take a significant amount of time to walk from one end to the other and that would be pretty boring in a game where you're routinely supposed to traverse the entire map to reach a quest.
I know I read something about the AC team researching and spending time in these locations in order to build them realistically, I'm trying to find it now. They've definitely done their homework while not sacrificing the playability of the game to realism.
Before AC2 came out, Ubisoft flew a bunch of game journalists to Florence to see how accurate the depiction was. You can check out some comparison screenshots here.