Sky trench

Sky trenches were a form of airborne defence used by the Time Lords. Much as a traditional trench served as a front-line defence to prevent enemy advance, sky trenches formed multi-layered armoured shields high in the atmosphere that prevented aerial invasion. (TV: The Last Day)

History
Sky trenches were built over Gallifrey in the early stages of the Last Great Time War. (AUDIO: Desperate Measures)

Almost nothing in the universe could get through a sky trench, and prior to the Fall of Arcadia, nothing had ever penetrated two sky trenches. (TV: The Last Day)

The Daleks broke through the sky trench protecting the moon of Tenacity. (AUDIO: The Conscript)

Bennus noted that none of Gallifrey's defences, including the sky trenches, would be able to withstand the impact of the Thousand Worlds which the Daleks planned to forcibly crash into Gallifrey at fifty times the speed of light. (AUDIO: The Heart of the Battle)

During the Last Great Time War, the TARDIS driven by the War Doctor bypassed the trenches while sensationally landing on his home planet. (PROSE: Engines of War)

Arcadia was protected by 400 sky trenches, making it the safest city on Gallifrey. Nevertheless, Dalek forces managed to break through these supposedly impregnable defences on the last day of the Last Great Time War, leading to the fall of the city. However, while many Daleks invaded the breach in Arcadia, the bulk majority of the attacking force could not descend into Gallifrey as well, failing to penetrate the trenches surrounding the exterior of the planet. (TV: The Last Day, The Day of the Doctor)

The Capitol was also protected by sky trenches. Despite gathering their full strength and bombarding Gallifrey continuously, the Daleks could not breach these defences, even at the very end of the Time War. "All thirteen" incarnations of the Doctor eventually saved the planet. (TV: The Day of the Doctor)

Behind the scenes
]
 * The "Sky Trenches of Arcadia" were the subject of a card in the online trading-cards game game Doctor Who: Worlds Apart.