Scent of Magic by Andre Norton

An orphaned child and captive scullery maid, young Willadene possesses an uncanny ability to sense and understand the magical odors that pervade her world. It is this remarkable talent–or curse–that carries her far from the fetid kitchen into an apprenticeship with a revered herbalist and untimately to the highest circles of the Ducal court. But there is depravity lurking within the castle’s walls, inspiring brazen treacheries and usurpations–and foul abduction as unthinkable as it is unexpected. Something horribly strange and forbidden struck at the dawn of a new day of corruption and terror. And an innocent girl finds the heightened sense that has been her fortune is now drawing her down into a maelstrom of evil.