A slave girl where it turns she's actually from a long line of royalty

I cannot for the life of me remember this book series. A high school friend of mine let me borrow it and I really liked it but besides a few key things I can't figure out what it's called.

The series is about a girl who is a slave and servant to the royal family but something happens and she has to escape alongside others but she winds up getting separated from her family. In one book she is captured and taken to a tower but I don't remember why or what happens in it besides she meets a man that she falls in love with. Along the journey she discovers that her and her family were never actually slaves at all but the original royal family and the original slaves usurped the throne and took over due to constantly being oppressed and treated unfairly. The last thing I do remember is the illustration of the last book and it's a girl in a pretty dress, I think yellow, on top of a horse that has short cropped hair that looks like a zebra's and a far distance castle.