In the 3rd edition the Astral void is connected to every plane and every layer of the Great Ring.
Does it mean that using a gate spell, a wish or a miracle you can arrive everywere (for istance right in the Gaping Maws, in Maladomini or in the Sixth Heaven of Celestia)?
So originally you get only to the first layer of a plane?
That's how I understood it, and why portals are so important... in the original 2e rules.
That's right, they changed how the universe works between editions. Shocking!
But the question is, if the astral plane exist only in the 1st layer of a plane, how do the outsifers manage to teleport themselves?
In the 1st edition there was a great rule about the nine hells about wish spells caste in one of the Nine Pits. Their effects were granted by the archdevil of the layer and he was able to block it or to grant it. That was interesting because it was a way to protect their plane by the invasors.
Well, the fiends used to do it thanks to *spoiler* the Maeldur ripping up the fabric of the planes for them, but now, who knows? I'd say that even on the lower planes, something like Planeshift, Gate, or Teleport could pop you up one layer with no control over the destination. And if you can teleport at will, it's only a matter of popping around three or four times till you hit the top, then you're off to wherever you meant to go.
That the Astral is connected to every layer of every plane does not mean that they are all accessible via the Astral. Astral Conduits only link to the first layers, and its easy to say that color pools also only link to the First layers (either because they just do or because highly responsible persons go around closing all the ones that open elsewhere to prevent stupidity).
Can wish, miracle, or gate get you to other layers? I'd say yes, and frankly, I'd say yes even if the planes were disconnected, since the whole point of Wish and Miracle is that they can break the rules. Of course, transporting yourself into certain areas via these spells is highly dangerous, as the plane itself may take your arrival rather unkindly. For example, wishing yourself into Avernus probably means you get hit by a fireball that very round, and the others layers of Baator and most lower planes have suitable nastiness to through around too. Also, such a massive display of power is likely to make the overlord of said layer or region (assuming that their is one) aware of what you just did, and they may choose, if they so desire to simply wish you back out again, probably to some place rather miserable. I mean, if you wish yourself into Nessus, Asmodeus probably knows instantly and wishes you right into the middle of a torture chamber. He who breaks the rules of the planes should fully expect to have the rules broken upon him in turn. More generally, the Quaruts are suppossed to prevent this sort of thing as part of their jobs, and CR 17 Inevitables are proabbly a pretty decent deterent.
Hmmm, my reply would be that if you're using the 3th edition interpretation of the astral as opposed to the 1st/2nd edition idea, and if there aren't special wards in the plane/building/area, and if you can handle the consequences of suddenly dropping into the most guarded places in the multiverse, then yes.
Traditionally though, according to the Planescape way of seeing things, no