Hmm... First thing I think of is Eiffel Tower. It's tall, it's Paris, it's old, lol. I wonder if you could somehow incorporate that. Lace and floral prints, aged maps... These are things I think of.
I'm going to just think out loud and ramble on a bit here...Square cakes. All covered in a creamy, antiquey, off white as the main layer on fondant. Bottom of the cake the bottom of the tower (seems like it might be not too hard to do this with the square shape of the cake; just add fondant to the corners in the shape/design of the corners of the tower), top of the cake the top of the tower, the tiers in between separated by elegant pillars. One layer with a 3D lace look to it-something like this maybe http://www.flickr.com/photos/22465667@N03/6220994649/in/photostream
Another layer with gumpaste roses and other timeless flowers done in soft but deep colors like purple, rose, greens, etc. You know, "dusty" colors. I think they look vintage, like an they've been aged but their true color is still under there peeking out.
And then maybe the 3rd tier up...oooooh oooh, I think I just got a good idea, lol. OK, on that tier, you have the basic creamy colored fondant and then you get images on sugar/wafer paper of small old posters from Paris and lay them all over the fondant or at least the sides for sure. It would be like it's wallpapered with beautiful old posters from old French plays and such.
OK, so I'm picturing this cake and it's awesome, lol. If you would need more than 4 tiers maybe just have a few extra simple layers in between the other decorate ones. Too much on each layer could turn out gaudy. Maybe I'll try to sketch this for you. I'll have to see how my sketching skills are doing today. Sometimes I can barely draw a stickman while others I do pretty well.
Let me know what you thin of this