The Abandoned Princess by Melanie Cellier

When Daisy dreamed of leaving home on an adventure, she never imagined ending up trapped in a tower, cut off from the world and invisible to others. The years pass slowly, leaving Daisy desperate to reclaim her old life, however dull.
But her captor has bigger plans, and Daisy knows she can’t just rescue herself. She has to shatter her captor’s enchantments forever. When opportunity comes, however, it arrives in an unexpected guise. Young and handsome, Prince Xander has come to rescue a princess from a tower. Instead, he finds himself battling at Daisy’s side for the fate of a kingdom.

In this reimagining of the classic fairy tale Rapunzel, the princess in the tower can only be seen by the one she loves if she first saves an entire kingdom.