The Crown of Thorns is an @Atlantean defensive network. An ancient relic from a long bygone era in @Atlantis, the artifact known as the Crown of Thorns is a vast protective network of deep mystical nature entwined with greater Atlantean science which has served as the ultimate defense from outside invasion.
The magic of such an apparatus is such that it can resist the force of crushing tidal waves and even an assault by some of the most powerful individuals the world has ever seen, the influx of magic from the Crown also has the effect of accelerating Atlantean adaptation to undersea life. The only way to enter and exit the city is through magical medallions for easy recognition by the barrier, but its effects aren't without fault.