Book of a Thousand Days by Shannon Hale

When Dashti, a maid, and Lady Saren, her mistress, are shut in a tower for seven years because of Saren's refusal to marry a man she despises, the two prepare for a very long and dark imprisonment.

As food runs low and the days go from broiling hot to freezing cold, it is all Dashti can do to keep them fed and comfortable. With the arrival outside the tower of Saren's two suitors--one welcome, the other decidedly less so--the girls are confronted with both hope and great danger, and Dashti must make the desperate choices of a girl whose life is worth more than she knows.

With Shannon Hale's lyrical language, this little-known classic fairy tale from the Brothers Grimm is reimagined and reset on the central Asian steppes; it is a completely unique retelling filled with adventure and romance, drama and disguise.