OK just before I start, I do know what does and doesn't constitute valid housing in Terraria. I'm also playing v1.2.4.1
What I'd like to know is does the "must have a door" requirement also mean "must be accessible", or can I create a walled-off cube with internal doors to satisfy the housing rules? i.e. are any of the houses A, B, C or D in the following diagram valid?
If I can, then I'm going to be making pressure-plate/activator automatic doors that don't get torn down in a blood moon.
Best Answer
Assuming that all of those have walls and a light source, those would count as valid housing. The requirement is that it has a door, not that the door go anywhere. Also notable is that wooden platforms count as doors for the housing rule, so you can actually just have a wooden platform separate the rooms from each other as well.
Here is an example of housing with internal doors:
This is actually a very common strategy to defend from a blood moon. As noted in the Base Defense Guide on the Wiki:
You can also just place a torch or other item in front of the door so that it can only be opened when you are facing towards the outside. From the wiki: