Quick question: Is it allowed? The rules don't seem to forbid it, yet again, from a thematical point of view it doesn't seem to fit (at least not in the first mission of the campaign) and on the other hand, tonight my very first action was spawning an Archvile inside the starting area and did pretty serious damage to the marines. So is there an official answer?

Try not to read rules that aren't there. There are rules for spawning, and if the conditions meet, then yes you can spawn. There are no rules for not spawning in the opening area. If the marines are foolish (or simply not experienced) enough to keep LOS covered in the starting area, then yes, you can that Archvile on them and watch them writhe.

If this is their first time playing, I probably wouldn't spawn so soon, just so they don't feel too discouraged...

-shnar

I still get goosebumps thinking about the one time I Darkness + Hell Knighted my friend in the start area . Silly sod wasn't covering both doors properly. He learned his lession. Of course, with the expansion Invader deck, could Earthquake them...