In The Sensorites story the TARDIS door's opening mechanism is a cylindrical electronic unit. When it's removed the door is permanently locked. Breaking down the door would disturb the field dimensions inside, something that the crew dare not do, even if they could.
In The Daleks the TARDIS lock is said to have 21 'holes': if the key is placed in the wrong one then the entire mechanism melts (An Unearthly Child). The Doctor says that he could make another key if given access to the TARDIS (Marco Polo), and he must have explained the technicalities of its use to Ian by The Rescue. However, the lock has a metabolism detector, which means that it will not work for anyone that the Doctor does not wish to enter (Spearhead from Space).
The Doctor's ring is also able to open the TARDIS doors (The Web Planet), and rights the damage inflicted on the lock by the Meddling Monk (The Daleks' Master Plan), during which period the Doctor removed the damaged original lock and replaced it with a simpler system. Thereafter the design of the TARDIS key changed quite regularly indicating that the Doctor upgraded its design every so often.
In The Mark of the Rani the Doctor uses his key to open the Rani's TARDIS which implies a degree of uniformity to Gallifreyan locks or a special modification to the Doctor's key.