[Sphynx]

1. Using the last technique, I create a wall with a number of faces and a less regular shaped portal that I want to cut into the wall - in this case, an airlock.
2. I repeat the technique to cut the hole in the wall - as can be seen, the boolean operation was this time less than 100% 'neat', but that's fine. The goldern rule in this is that we want points, not polygons.
3. A cleaned up mesh, with some points joined together, and the original polygon arrangement modified to my liking, not Blenders.
4. I select the inside rim of the hole and extrude outward (I don't need to create a face across the hole for this one, as I can extrude along a normal axis) to create a door rim, then again to get some door depth.
5. I select two of the perfect opposing points of a half-circle to establish the centre for scaling this side of the door (so that the circle scales smoothly), and put the cursor at that point. I then select the points that make up that curve of the door and scale to 90%.
6. I repeat the process with the other half of the door, then separately with the two centre points as they were not part of either circle. I now have an angled recess in the door, giving the impression of bulk.
7. I repeat the process of extruding and scaling demonstrated over the last few sections to create a recess in the front of the door.
8. Now all we need are some details to give the door some life.