You can unlock it in Royal Arena (Arena 7)
Long range troop with are damage and low hitpoints.
With very long range she can can target and shoot Arena Towers from the opposite side of the river.
Good counter for hordes of low hitpoint troops.
To kill the Princess is good spawn Giant or Knight in front of her because of her low hitpoints.
If you place Fireball, Arrows or Goblin Barrel on the right spot you can eliminate her easily.
When defending against an enemy rush, deploying her at the back of the Arena is beneficial, as her long range will allow her to attack while receiving no incoming hits.
Keep in mind that Spells can still reach her.