Due to their immense power, Espada of Rank 4 or higher are forbidden from releasing their Zanpakutō within Las Noches. It is later justified by the battle between Ulquiorra and Ichigo, as their fight above the dome resulted in Las Noches losing a good portion of its domed roof, as well as segments of the five towers. The Gran Rey Cero is so overwhelming, that one of the only two rules the Espada have,is that, that they must never use it underneath the dome of Las Noches as the sheer force of just using can cause Las Noches to be destroyed.