AC3's maps are nowhere as simple as a user created map in a game. The developers actually do their research which includes going to those places and trying to be accurate to the setting and time period, including inserting recoginzable landmarks and buildings. That takes time and not merely an "easy" task
Trust me. It's still a LOT easier than creating the underlying engine. As a matter of fact, someone with pictures/illustrations of early France, access to Ubi's SDK (software development kit), and a lot of patience can probably do a good approximation of the city.
As long as the engine has established the proper "materials" for the world, such as "this is dirt and this is how it behaves when these things occur" or "this is tree branch variance A and this is how it acts when a rope is thrown across it" etc., etc.
As long as all of the above is established, everything is else is window-dressing on top of it.
You can change the textures of "bricks" to better match what they would look like in France, while the frame underneath it is the standard framing they use for buildings in the game, which establishes what walls the playable character can run up on, what handholds he can hang onto, etc. etc.
As big as Connor's world was, you can easily enumerate and recognize the number of items he was allowed to hang onto or climb onto.
I mean, even the Arc de Triomphe is based on Roman architecture, which Ubi has tackled quite well already.