Siege mode was a feature of the Doctor's TARDIS that could be activated using a lever under the console to prevent entry or exit. Additionally, it could protect the TARDIS when its shields were compromised. Siege mode required a sufficient amount of power to activate or deactivate.
The Twelfth Doctor activated it when his ship was in the path of an oncoming train with no shields. While in siege mode, the Doctor's TARDIS no longer had the appearance of a police box, but of a cube with Gallifreyan symbols along its sides. (TV: Flatline)