What is the symbolic importance of the Eiffel Tower in the novel, and in Émile and Cait's relationship?

Discuss this conversation between Cait and Émile, about the Eiffel Tower:

"But the fact is that it is not trying to be anything rather than what it is. Nothing is hidden and the reverse is also true; nothing in the city can hide. From the top on a clear day, you will be able to see everything. It will all be gloriously transparent."

"It's what we want, isn't it?" she said. "Transparency. One so rarely finds it."

What is the symbolic importance of the Eiffel Tower in the novel, and in Émile and Cait's relationship?

My initial, smartalecky, response to this question was that of course the Eiffel Tower is a gigantic phallic symbol. It’s a metaphor (analogy?) that can be extrapolated to be meaningful within Emile and Cait’s relationship with each other as well as within the context of the book. Reading this book I found it more than ironic that the Tower’s erection (pun intended) took place during a time period where women were seeking greater autonomy and beginning to press for the right to vote - on both sides of the Atlantic.

The line where Emile says, “on a clear day you will be able to see everything,” cracked me up. What I read in his remarks was that from the top of this giant penis you will be able to see everything from a man’s point of view! I’m sorry if anybody takes offense at this but there is such a push and tug between old/new, class system/freedom, male/female, traditional/modern that I applaud Ms Colin’s ability to bring all of that into play.

Well, I'm not sure I saw the symbolism quite as explicitly as donnac, lol, but yes, I do think the Tower is meant to show the changes coming in aesthetics, society, politics, class divisions, male-female relationships and that the changes were being reflected in the relationship developing between Cait and Emile--the explicit sexual changes seen by donnac came many years later--universally, at any rate, though those bohemian Parisians were way before the rest of us.

I must say, the phallic symbol suggestion from donnac is very interesting and one that, now pointed out, I totally agree with. However, when I read the novel, I have to admit, that imagery didn't cross my mind - I was completely wrapped up in the emotional trials and triumphs of each character.
For me, Emile's comment about the tower's transparency is the personification of Cait's personality and exactly what Emile wishes he could achieve for himself and his relationship with Cait. Cait is obviously not your typical female of the day. On the one hand, she's very transparent (even with her internal struggles and secrets). What you see is what you get. I think because of her age and life experiences, she seems a little tired of the social expectations of women and the social standards she's expected to live up to. However, she is forced by society to keep secret the single most important relationship she's had since her husband's death. The one thing that's made her feel alive again must be hidden and treated like a dirty little secret. From that required secret, I feel Cait's transparency in other areas of her life get muddled and because of that, she sets on a course of struggle and internal conflict that ends up affecting other areas of her life.
Emile is a character in constant struggle with who he is, who he wants to be and what others expect him to be. Unlike Cait, he seems to always struggle to decide just how transparent he really wants to be. The tower represents so much more to him than just an architectural phenomena. With all the transparency he's chasing in life and career, he still must hide his relationship with Cait behind the walls of proper etiquette.
Cait's comment about transparency and it's rarity reflect what they are both feeling in who they want to world to allow them to be.