Transduction barrier

The transduction barrier protected the Time Lords' planet of Gallifrey from other species. It seemed to effectively function as a force field.

The barrier was deactivated on the orders of the Fourth Doctor. This allowed the Sontarans to invade Gallifrey. (DW: The Invasion of Time) A squad of Daleks that infiltrated Gallifrey lowered the transduction barrier, allowing them access to Gallifrey to invade. (BFA: The Apocalypse Element)

In an alternate timeline, the Knights of Velyshaa succeeded in breaching the barrier and invading Gallifrey. (BFA: The Sirens of Time)