Duel Arena was the safe place for PvP action. Because of the several updates of Insanity now it's possible to duel everywhere. If you want to duel someone, make sure you are a little distance from that player to get the option and challenge him/her to duel.
The Arena is located in Saint Morning, South of Saint Hall. It is a large area where players could battle against eachother to gain a PvP title.
For more information, please visit the PvP page.