A defence shield was the primary defensive capability of a TARDIS. Following his tinkering with it, the Fourth Doctor was unable to repair his TARDIS's defence shield. He was, however, able to extrude the shield on the door as that was on a different circuit. With this he created a breathable atmosphere between the TARDIS and a Skonnan freighter. (DW: The Horns of Nimon)
The Fourth Doctor, Romana and K9 pass through the defence shield of the Doctor's TARDIS. (DW: The Horns of Nimon)