Is it possible for a powerful unique outsider, like a duke of hell or a consort to an archdevil (like Lilith or Lilis) to enter in Sigil?
Could, for istance, princess Glasya have entered in the Cage before becoming an archdevil?
And what about a real archfiend or a divinity of divine rank 0?
I am doubtful even that would provide a good assessment of Her power. Remember, the Lady is a force that utterly destroyed a greater power, Aoskar, when he tried to wrest control of Her city from Her. That in and of itself signifies incredible power. If one subscribes to the ideas in "Die, Vecna, Die," it could be argued that there is a force in the multiverse that can stand up to Her. Personally, I have a rather dim view of DVD but that is my own opinion.
As far as I am aware, archfiends like Graz'zt, Demogorgon, and the Lords of the Eight cannot enter Sigil, nor can celestials of similar power. A balor or a solar could, but that is probably the upper limit.
A fiend could certainly enter the Cage prior to crossing the threshold. However, once that fiend does cross the threshold it is likely that it will be "encouraged" to leave or suffer the consequences.
Rare would be the fiend that achieves sufficient power to attract the Lady's attentions while within Sigil, though. It is certainly possible, but probably easier to accomplish outside the Cage.
The Savage Tide adventure path in Dungeon magazine mentions that Gwynharwyf, Orcus, and Malcanthet are unable to enter Sigil.
I see.
Gwynharwyf, Orcus, and Malcantheare a celestial paragon and two demon lords, so it is quite probable that the lady wouldn't let them pass.
And what about the Glasya described in the BoVD? Her CR is 18, she isn't very powerful.
I ask this because is very importan for my adventure 
As always, do whatever you feel fits the campaign and try to be consistent. The material isn't very specific about the issue, so whatever you thinks makes sense will be the correct answer. Still, the CR of a person would not be an indication of the status he or she holds. I think in Planescape that especially holds true. Thus, if you rule that Mephistopheles wouldn't be able to enter Sigil, then logially Glasya wouldn't be either, not matter her actual power.
I just have this issue in my campaign as well. I ruled that none of the lords can enter Sigil. No exceptions.
Well, the reasoning they gave in Savage Tide was that the Lady wouldn't allow anything with worshippers, which rules out most outsider paragons.
As I recall, pit fiends, balors, planetars and solars are also unable to enter Sigil, seeing as their power level verges on the epic, which is why you often find mid-ranking fiends and celestials acting as (non-divine) proxies for their high-ups.
It's definitely not that far; there's a few references in PS products to creatures of those types in Sigil. The only one I know offhand, though (since I was just reading Faction War), is the pit fiend that it's said controls Hellgate in the Lower Ward.
Well, the whole point of Faction War is that the portals have gone crazy and the fiends are invading, but I don't remember where my beliefs arise from.
No, he was before, not during. He was mentioned in that section before the actual adventure where it gives a run-down on areas and places in Sigil.
And there was the solar who desceded from the 7th heaven of Celestia and came to Sigil to save a lantern archon too. It was an adventure in planes of law, if I recall well.
BTW, solars and pit fiends doesn't have worshippers, usually. They should be able to enter, if what Spiteful Crow said before is correct.
Quite why Galgaliel can enter Sigil is another matter. If the heroes rescue the latern archon, he can alter alignments towards good, bestow paladinhood on LG fighters and other such near-divine acts. What's more, he's the only creature in the Planes who has ever returned from the coruscating energies of Chronias.
If the only difference between him and a god is that he isn't worshipped, why doesn't the Lady block him from entering Sigil too?
According to In the Cage, only powers are explicitly barred from the city. Even proxies are allowed in the city, and if In the Cage is anything to go by, there are a good many proxies in Sigil. Given the power most proxies have, I would say that balors, et al, could indeed go into the Cage.
I do not recall reading any specific notes about barring fiendish or celestial lords from Sigil, but I would probably rule them out for my games. Graz'zt, for example, is all but a power.
Unless it is written in a book somewhere that pit fiends, etc., cannot enter Sigil, I would wager that the reason you see fewer of them is because there are fewer of them and they're usually busy doing other good or evil deeds elsewhere.
I don't have the adventure on me at the moment, but I can say without a doubt that the PCs are hired by either a Balor or a Pit Fiend who is in a bar in Sigil at the beginning of the 2e module Into the Abyss. There's also mention of a Pit Fiend and maybe a Solar in another bar in the intro to The Deva Spark if memory serves.
As some of the previous posters stated, I've always believed that the actual prohibition of powers entering Sigil extends only to Gods and things of similar power like Layer Lords. However, the fact that no one knows where The Lady draws the line means that a good number of more powerful beings will probably avoid Sigil anyway, just in case.
I generally interpreted the restriction as a matter of power of influence within the city. So a Power, or one of the Abyssal Lords? Definately a no - they're too disruptive to the city balance. In a simular way - it is possible that a powerful pit fiend commanding legions of troups may just be too influential if it were allowed in. As would be a 50th level epic wizard from the Prime who likes turning everyone around him into undead by snapping his fingers... but the same wizard in terms of stats with a different personality that just wants to pass through the city on his way someplace else? That'd be fine.
Forbidding the higher level creatures or outsiders in my game would boil down to the danger they pose to the city if allowed to travel freely there.
This is simular to how Shemmy runs it in his game (which made it questionable at one point if my PC would be allowed back in the city - so far so good.) There's been at least one NPC in his game that was thrown out of Sigil because he spread his political influence farther than he should have - he didn't need to be a Power to get exiled.
I think I'd allow it. Glasya before she became ruler of Malbolge and Glasya after she became ruler of Malbolge are very different matters. I think I'd allow exiled former rulers like Moloch, too. Now that Moloch has lost the power he once had as ruler of Malbolge, he can only appear as a "private citizen" - powerful, to be sure, but if he's not disruptive he might be permitted in.
If you decide that Glasya and Moloch have cults of worshippers that's a different matter. Then they're essentially powers, and it's best to keep them out.
But what about aspects? They're generally much weaker than avatars. Could a CR 10 Aspect of Graz'zt enter the City of Doors?
Thank you Ripanwormer
That's the point. Glasya didn't have any cult until some years before her rise as an archduchess. She formed them slightly before her ascension.
I also believe that a consort or a duke should be able to enter in Sigil, but not those that, like Lilith are worshipped like gods.
I don't remembrer if Moloch still have a cult on the prime, but if it is true, he should still be banned from the city.
Speaking of aspects... well, an aspect is different from an avatar. They are NOT the same person like the 19 avatars of Corellon, for istance. An aspect of demogorgon is one creature formed from a powerful outsider's essence, that is connected to it but is still a separate creature. (there are also divine aspects, but I don't use them in my game,I just prefere to use avatars). If you kill an aspect you make its "boss" weaker (it is described at the end of the "Savage Tide" adventure, with Demogorgon), when you kill an avatar, it is all another matter.
The lady of Pain should not allow aspects in Sigil only if you think that they are something more then cleric. Are aspect connected to the energy of the worshippers? I'd say yes.
I generally interpreted the restriction as a matter of power of influence within the city. So a Power, or one of the Abyssal Lords? Definately a no - they're too disruptive to the city balance. In a simular way - it is possible that a powerful pit fiend commanding legions of troups may just be too influential if it were allowed in. As would be a 50th level epic wizard from the Prime who likes turning everyone around him into undead by snapping his fingers... but the same wizard in terms of stats with a different personality that just wants to pass through the city on his way someplace else? That'd be fine.
Forbidding the higher level creatures or outsiders in my game would boil down to the danger they pose to the city if allowed to travel freely there.
This is simular to how Shemmy runs it in his game (which made it questionable at one point if my PC would be allowed back in the city - so far so good.) There's been at least one NPC in his game that was thrown out of Sigil because he spread his political influence farther than he should have - he didn't need to be a Power to get exiled.
I think that the Lady would prefer to exile a dangerous mortal in the Mazes more than preventing him from entering in Sigil.
After all she allowed Duke Darkwood to walk in Sigil, and well he was able to create many problems, you know... even in the past when he became a very powerful epic wizard.
She has plenty of options available - Mazing, Flaying - are usually punishements for when you step over the line. But if she can avoid the problem by just forbidding you from entering - after all, the only 'crime' you would have performed was existing - I could see that being a viable approach as well. (And it does say something about the power levels we're talking about in Shemmy's game if Darkwood didn't qualify to get kicked out. The NPCs he reserves Exile for are the ones that in his game make Darkwood look like a child playing with paper dolls.)
Wait, is this Clueless we're talking about or someone else?
Anyhow, it makes sense for 'problem children' to be barred from entering Sigil in the first place, since that stops any possible issues with no harm to the city.
An interesting question would be, what about a ruler who is declared deified by his people and has a cult around him and exerts influence on the planes?
Ah... no. 
though his playing around with Heavy Magic came close. You did meet an example of this sort of NPC in Storyhour One: The Jester (The creepy one with the maze under the Jester's Palace, not the half crazy Ringgiver.)
And Velk, Clueless's kid is coming very very close to getting tossed out - hasn't happened yet, but depending on which way the story goes whenever that game picks back up - I wouldn't be surprised.
As for a deified ruler? Like the Pharohs of Egypt? Well - belief is belief, and if they have enough of a kingdom population to be actively worshipped in a cult? Then I'd think they count as a demi-god at least... so - no Sigil unless they pull the same sort of stunt that Finder Wyvern-whats-his-name did. Leastwise that's how I'd call it.
Heh, if the standard is being worshipped then it's a good way to get rid of your annoying neighbour- next time they're out of town set up a small shrine to them and offer regular sacrifices. They'll never be able to get back.
Oh, and the celestial paragons don't accept worship according to the BoED. They refer those who try onto the appropriate deity.
Wait, is this Clueless we're talking about or someone else?
Anyhow, it makes sense for 'problem children' to be barred from entering Sigil in the first place, since that stops any possible issues with no harm to the city.
An interesting question would be, what about a ruler who is declared deified by his people and has a cult around him and exerts influence on the planes?
Why doing it when she proved more than once to be able to Maze Everyone she wish in one moment? Why preventing a person to come inside Sigil, when she allowed Galgaliel to step in?
And if you ever find a way to escape, she can send you thousand years in the past!
Btw, the Duke Darkwood that was imprisoned in the gem at the "end" of the adventure Faction Warshould be a reallypowerful NPC wizard... a real one if you think that the Sigil spell he created should be something like a 12th level spell (never casted actually, but working) or an epic spell of abnormous power.
Why not?
Ah... no. though his playing around with Heavy Magic came close. You did meet an example of this sort of NPC in Storyhour One: The Jester (The creepy one with the maze under the Jester's Palace, not the half crazy Ringgiver.)
Yeah I definately got that The Jester came very, very close to getting owned by the Lady of Pain if he hadn't left Sigil when he says he did.
I was actually thinking of the Romans of the Imperal era, who got the deification tradition from the Greeks and Eygptians, so it is similar in. Being very polytheistic, they had these small cults to the household gods and the ancestors, but one eventual sprung up around Ceasar while he was still alive and it became very powerful. Eventually offerings would be made to the spirit (they called it genius) of the current Princep, like offerings were made to the head of each family, and there were small rites that people would partake in like the offering of insense in the name of the Princep. They were deified after death, so they would become a true god then. That type of belief on the planes would be pretty powerful.
Why doing it when she proved more than once to be able to Maze Everyone she wish in one moment?
See, that's something I've always wondered about. If she could Maze everyone she wanted in one fell swoop like that, why did she create the Indep disease? I mean, Factol's Manifesto said she was actually worried she wouldn't be able to control the population with such a huge Indep population. And that wasn't from a character's perspective, that was DM backstory info that was presented out of character, and thus not susceptible to an unreliable narrator effect.
The Factol's Manifesto is all written as though by journalists so it's plausably fallable 
Also it's possible that, if the mazes are still in the cage, the Lady wanted to decrease the power of the Indep beliefs in the cage and that could only be done by killing them.
The Factol's Manifesto is all written as though by journalists so it's plausably fallable
It's not all written as if by journalists, though, just a good portion of it; I can't see why an in-game work would have a section headed "The DM's Dark", and the content certainly doesn't make sense for a book like that, so I always assumed that whole section was something that didn't exist in the in-universe version of the work.
Also it's possible that, if the mazes are still in the cage, the Lady wanted to decrease the power of the Indep beliefs in the cage and that could only be done by killing them.
We already know for sure that they're demiplanes in the Ethereal, though, so that can't be it. :/
In 2nd Edition, the line between powers and non-powers was moderately easy to draw. You either were a creature with hit points, or you were a divinity, powerful beyond the measure of hit points. So solars and pit fiends were fine; demigods weren't. Since the Lords of the Nine and beings like Orcus weren't given stats, the easy assumption was that they were Powers (if not "true gods", whatever that meant), and barred from Sigil.
(This made additional sense when you went back to First Edtion for guidance. Deities and Demigods said Demogorgon, Juiblex, Orcus, Yeenoghu, Asmodeus, Baalzebul, Dispater, Geryon, Bahamut, Tiamat, Lolth, the Elemental Princes of Evil, Ssendam, and Ygorl were lesser gods. I seem to recall there was a later listing that covered some of the uniques in the Monster Manual II, but I can't find it.)
For 3e, anybody without a divine rank isn't a power. Anybodywith a rank of 1 clearly is. 0 . . . is tricky, isn't it?
Well, the prevailing theory is that The Lady blocks powers somehow. She's the one who draws the line, so what power level is she comfortabe with? I suppose you could find out how powerful Her Serenity is by carefully documenting the most powerful individuals she allowed to enter Sigil, and not get mazed.