"The House that Jack Built" was written to focus on Emma Peel, allowing Patrick Macnee some time off by only featuring him in a few scenes. The opposite was done on the "The Girl from Auntie" (4.17).
It's revealed that Mrs Peels maiden name is Knight.
"The House That Jack Built" inspired "Murderworld," an assassination by amusement park themed deathtraps facility in the Marvel Comics universe; in particular "The Uncanny X-Men" title.
The plot point of machines operating that were built by someone who is now dead was also used in Sky Captain and the World of Tomorrow.
The iron spiral staircase Emma descends was seen earlier in "A Touch of Brimstone"