Trenzalore force field

The Trenzalore force field was established by Tasha Lem's Papal Mainframe to protect the planet Trenzalore when a message broadcast from planet, plunging the universe in fear. As a result, no spacecraft which entered orbit could land on the planet, including the Eleventh Doctor's the TARDIS.

Meeting with Tasha, the Doctor and Clara Oswald were granted permission to land, just outside the town of Christmas, in order to find the source of the message. However, Tasha forbade the Doctor from bringing technology, not even his TARDIS key. On the planet, the Doctor and Clara found a group of Weeping Angels who, as the Doctor observed, had evidently made it through the shield. Using a spare key in his quiff, the Doctor was able to call the TARDIS to retrieve himself and Clara.

300 years into the Siege of Trenzalore, the Daleks attacked the Papal Mainframe and killed Tasha several times in order to gain her knowledge of how to break the force field. Tasha warned the Doctor that the force field would hold for a while but that there were already breaches and the shield would eventually decay. (TV: The Time of the Doctor)