At the center of Seireitei is the large white tower of the Senzaikyū, which is linked to the Sōkyoku Hill where executions take place using the Sōkyoku (双殛, Twinned Punishment). The walls of the Shishinrō (四深牢, Four Deep Prison) within the Senzaikyū are made of Sekkiseki (殺気石, Spirit Reducing Stone). Its presence interferes with the prisoners’ ability to sense Reiatsu. In addition, the constant exposure to the mineral drains their Reiryoku, reducing it to its absolute lowest level. This leaves them particularly vulnerable to powerful Reiatsu.