As far as I understood one of the Tuesday videos about A15, the enemies should only spawn outside the convex hull of your total settlement. I remember that mining areas and buildings were mentioned to affect this convex hull, but do placed items count, too? I came across something, that I assumed should not be possible.
Please note my placed fences and braziers. My settlement is to the right of this view, though goblins spawned basically right on my fence, undead inside what I declared to be my land:
Still same situation, just more zoomed out. In the bottom right you can see my furthest building, on the terrace steps above (but on a similar distance) are the furthest of my streets (which are also considered as buildings).
This is the same area, but actually the night after. A new batch of undead spawned across the terrace steps (one skeletton can be seen on top, next to the green attack marker, some more zombies spawned in the forest below, near the red marker, but already started their attack on my village).
This was release-549 (x64) on Steam, Windows 10 x64, no mods.