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.