Sky trench

Sky trenches were a form of airborne defence used by the Time Lords. 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)

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. (TV: The Last Day, The Day of the Doctor)

The Capital 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)