Keys to the Kingdom/Fridge

Fridge Brilliance:

 * The Denizens of the House are angels. Or at least as close as you can get, without actually calling them such. They're immortal servants created directly by God at the beginning of time to observe and maintain Creation. The tendency of at least the higher-ranking Denizens to be Winged Humanoids that are supernaturally tall and beautiful, with auras of authority and power is only fodder. This leads to:
 * Lord Sunday is probably the closest in-series equivalent you can get to Lucifer. He's the most powerful Denizen/angel who lead the rebellion against The Architect/God. The seven trustees who betrayed The Architect are the fallen angels who followed.
 * The start of all Creation turns out to be
 * The Old One and the Will's description of how to use the Improbable Stair is the same because
 * In the earlier books, Lady Primus is always talking about how Arthur or herself must use the keys to patch up the holes that Nothing is seeping through in various parts of the house. It apparently didn't work, despite her splitting herself in two, but it all makes sense by the last book because
 * Some of the sins that go with each day match the names very well.
 * Thursday's sin is Wrath. "Thursday" is a corruption of "Thor's day," who is shown to have anger issues in the myths.
 * Friday's sin is Lust. "Friday" is "Frige's day." While not much information on Frige survived Christian purges, she is generally agreed to be a sex goddess.
 * Sunday's sin is Pride. "Sunday" is obviously "Sun day," which brings to mind Icarus, and it is also called "The Lord's day."
 * The others don't fit as neatly, but might have inspired the author. For example, Monday is "Moon's day," bringing to mind night and sleep, which implies sloth.