Script editor David Whitaker rewrote the draft script to remove the Doctor from the plot, to allow William Hartnell a week's holiday. This is why Hartnell is credited, but does not appear.
David Whitaker came up with the idea of a house full of traps. Terry Nation largely transferred this to an exterior setting because the first two episodes had already mostly taken place indoors.
Guest Star Edmund Warwick, would later appear uncredited doubling for William Hartnell in Episode 3 of the Dalek Invasion Of Earth, after Hartnell had been injured, he would then later double for Hartnell again in Episodes 4 and 5 in shots where Hartnell would play the Robot Doctor, a role he shared with Warwick.
Contrary to popular belief William Hartnells credit was not because of a late re-write which is why he's still credited but doesn't appear, it was in fact in his contract that he was to always be credited on the show even if he didn't appear, this same clause however didn't always extend to the other regulars.
This is the First Episode of Doctor Who to be broadcast on BBC 1, whilst the previous 22 Episodes were broadcast on BBC, during the previous episode and this one the BBC was given a second Channel BBC 2 and as such Doctor Who's home became BBC 1.