Transduction barrier

The transduction barrier protected the Time Lords' planet of Gallifrey from invasion. Effectively, it was 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 which 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 breached the barrier and invaded Gallifrey. (BFA: The Sirens of Time)