Symbolism, Imagery, Allegory
When Newman first encounters the vast walls of the Carmelite convent where Claire plans to live out her days, he's totally puzzled. He thinks of the convent as a
"[…] page torn out of a romance, with no context in his own experience." (24.1)
Newman's never really fully comfortable around all of the impressive European architecture, so it makes sense that he doesn't feel like the convent is really real.
Newman also keeps projecting his fantasies onto Europe, only to find out that he's basing his ideas on romantic ideals. Move to Paris to find the perfect wife? Good luck with that, Newman. The convent represents everything Newman thinks is cut from the cloth of fairytales…only it's not.
Instead, the convent is the place where Claire becomes completely unattainable.
The Biggest Barrier
Once Claire becomes Sister Veronica, there's no hope for Newman of a happy ending. But since he can't confront Claire in person, he mopes around the Carmelite convent:
"[…] a dull, plain edifice, with a high-shouldered blank wall all around it." (26.11)
So the convent's gone from becoming a romantic ideal to something totally blank. Oh, that's bleak.
Of course, Newman's changing views of the convent could also suggest that he's developing as a person. Maybe he's not as confident in having a Disney-fied life, but he seems to approach the world more thoughtfully.
Who knows, maybe Newman will even recognize that Claire's choice to leave him is a valid one. Yeah, probably not.