My Fair Prince(ess?)

When a small country starts to lose it's ground in a war with the biggest empire of all time and the most ruthless emperor ever wrote in their history, they are given two options. Hand over their crown prince, or face defeat and invasion. What's an emperor to do? Hand over his only son and first in line to the crown, to a man who's known to enjoy torture in the daylight as well as the dungeons, or face dethroning himself?
Neither. The solution was so simple, hand over his illegitmate daughter disguised as the prince, their looks were almost identical. She was already a lowly servant, what difference could it make if she were to look like a boy and be a servant somewhere else? If she were to die, no one would miss her. It was the perfect plan. So he thought.