Well, after another three games with it, here's my feedback on the base I've built.
In S1 - 2 for aliens, the base is near impregnable. Putting a few turrets in the "back" of the base with the armory means even if a 'rauder jumps it, he's still going through a hail of fire.
However, as with all good bases, the turrets can't be too far spread out (anything beyond the grey concrete under the stairs is TOO FAR)
At S3, the base becomes VERY snipable (I discovered to my horror that a rauder or a adv goon can actually sit on the stairs at a certain angle and snipe my reactor) but no tyrant will live if it rushes in blindly.
So with that in mind, while not the *best* base on the map, it's still good for if you were perhaps rushing for the elevator room but a dretch is in the way.
As to the dark area under the stairs in the first room "upstairs" on karith
That could work. If you put the reactor, armory and node behind the wall, it becomes unsnipable and a single tesla could adequately defend the whole thing. There's no room for jumping on top as a 'rauder either.
Also, the "entrance" to that corner has boxes stacked up so you can hide turrets behind it to make it even tougher to get in to.
Again, however, this base is VERY pouncable and snipable (Although humans can benefit from this too of course)
What I don't like about both this base, and my own above, is that there is a place for aliens to retreat to if they get hurt. In both cases, all the aliens need to do is jump on top of the respective walkways and they're pretty much safe. It also means any unwary human running out could possibly get head-chomped easily.
In all honesty, despite how I dislike the elevator room as a sort of deathtrap dungeon, it IS also the most eminently defensible and efficient. In S3, you could set up a wall of teslas behind the door and no tyrant or 'rauder is gonna jump or slash past it, and no adv. goon is going to be able to snipe from afar since he needs to open the door (and thus get zapped) first. Taking down that elevator room base will def. require alot of teamwork too (And, of course, no tyrants since they can't jump up on top....)