I thought the catacombs were partially a result of the plague?
Also, there's a crazy documentary just about Paris sewers, and how they've had to adapt over time. Not only do you have the catacombs, but you've got miles of old sewer lines, giving you a setting very much like that of underground chicago in Dresden.
http://itotd.com/articles/432/paris-sewers/There's one article about it, at least.
I also thought about white court presence and the city of love, but it just seemed too easy... perhaps easy is good though.
Hrm... the champs-elysees used to have a white-light only ban, causing it to be the only place in the world where mcdonald's golden arches were white. I'm not sure if the ban still holds, but it's proximity to elysee castle (and the elysian field mythology, "where heroes go to relax") could mean that the 1 1/4 mile strip of white light could be a calculated occult development... perhaps serving as a sort of "landing strip" or "take-off strip", launching certain beings into the ether. The veil between worlds (particularly heaven?) could be treated as 2 less in that area, giving angels and the like a more powerful way into the city. I mean, sure, churches work like that too probably, but come on, why would heaven have NEEDED such a large landing strip?