I'm looking for the name of a fantasy book.
Interesting question - thanks for asking! One of those fantasy plots that sounds really, really familiar but could be one of about a thousand! I've got two possibilities for you, and then a series of questions to help us narrow it down.
My first and best bet: Mercedes Lackey, To Light a Candle. It's got the city of mages, two very different and conflicting societies, and a snowscape on the cover.
Second, not as likely but I'm honestly buying myself time to think - Walter Dean Myers had a (comparatively) obscure fantasy book called Shadow of the Red Moon - in that one, there's a comparatively advanced city protected by a magic (or lost tech) wall that's besieged. The edition I remember reading had a very distinctive black-and-white woodcut cover, which is probably why I'm thinking of it now.
Third possibility, a great read even if it's not quite the one you're after, is China Mieville's The City and the City. It's got a muted cover and two cities (actually occupying the same space, shifted apart through dimensions) sealed apart by powerful magic but with tensions rising between them. It's not a high-fantasy book - Mieville has kind of an interesting touch, it could almost be a mystery novel or sci-fi.
To help us narrow down the search for the book, do you happen to recall how recently you read or heard about the book? Would you say it's closer to two, five, or ten years old? Is it a medieval-ish fantasy world - knights, castles, 'traditional' wizards? And do you happen to recall anything about the primary protagonists of the novel - does the story focus on a young woman or man, are they part of an adventuring group, can they use magic?
I'll keep thinking about it overnight - but feel free to email back or call 402-444-4800 with additional questions!