Sky trench

Sky trenches were a form of airborne defence used by the Time Lords. Much like a traditional trench would be used as the final line of defence to stop an enemy from advancing into another's territory, sky trenches also served as an ultimate line of defence, forming a series of multi-layered armoured shields high in the atmosphere, designed to block off the skies from aerial invasion.

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)

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, the Daleks were never able to breach these defences, even at the very end of the Time War. (TV: The Day of the Doctor)