The Forbidden Zone was the defence area around the entrance to Central Control on Earth.
It contained hidden surveillance cameras and a lethal, high intensity radiation grid covering an area about 50 yards around the bunker entrance. The grid could apparently root people to the spot, preventing their escape, and was self-repairing in eight seconds.
Arle and Berg died in the Forbidden Zone after stepping on the grid. Blake, Vila and Gan were able to get through due to Avon shooting at low level to give enough time before the system repaired itself. When Avon attempted for himself, he tripped but Blake was able to shoot again before the grid connection sealed itself.
Servalan used her authority to have the grid deactivated, allowing Travis and two mutoids access to the bunker. She later followed herself, held at gunpoint by Jenna and Veron.
Avon supposed that the bunker might also be fitted with antimatter screening. (TV: Pressure Point)