TARDIS-proofed. And actually so, if even three Doctors couldn't get in without an alternate back door (well, painting).
How? It seems unlikely even with UNIT's increased access to alien tech (and alien advisors other than the Doctor, if the Vlinx is any indication). Did the Doctor themself work on such a system (perhaps to prevent the Master materialising in UNIT HQ), only to have it turned against them?
Has it ever been revealed in novels/EU material?
You'd think the Daleks etc would be VERY interested in stopping the Doctor popping up unexpectedly.
How? It seems unlikely even with UNIT's increased access to alien tech (and alien advisors other than the Doctor, if the Vlinx is any indication). Did the Doctor themself work on such a system (perhaps to prevent the Master materialising in UNIT HQ), only to have it turned against them?
Has it ever been revealed in novels/EU material?
You'd think the Daleks etc would be VERY interested in stopping the Doctor popping up unexpectedly.