Transduction barrier

The transduction barrier was a quantum force field (AUDIO: Prisoners of Fate) that protected Gallifrey and the Time Lords from invasion.

These barriers could also be raised around Time Lord space stations, and TARDISes were unable to pass through them. (AUDIO: Trial of the Valeyard) One TARDIS did break through, but had to jettison 9/10ths of its interior, and most of its circuits fused while its recall circuit burned away, leaving only the telepathic circuits functioning. (AUDIO: Prisoners of Fate)

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

In an alternate timeline, the Knights of Velyshaa breached the barrier and invaded Gallifrey. (AUDIO: The Sirens of Time)

The Eighth Doctor breached the transduction barriers to land the TARDIS outside Madame President Romana II's presidential palace. (WC: Shada)

Anti-Time was unable to pass through the transduction barriers. (AUDIO: ''Neverland)