Planescape continues to interest me, and if you look far enough you can get there from Castle Nicodemus.
My Sigil is a place of wonder and amazement. My Sigil is a place of disillusionment and horror. You will not find priests and priestesses there energised and fresh from afternoon tea with their deity. You may, however, find clerics who seem to stare blankly through you, and who never say a word.
There is no vast wilderness where a hexcrawl will eventually take you from Asgard to Olympus to Gahenna. There is no miles-high spire of rock with a spinning cheerio at the top. There is, though, a great and ancient archway which holds a door of iron 101 cubits high, and which bears the words "Lasciate ogne speranza, voi ch'intrate."
In fact, you will never see the 'outside' of Sigil's massive toroid. Assuming an outside even exists, in it you would find yourself outside reality, and it would shred your mind.
Sigil is not the true name of the city. This moniker refers to the symbol which stands for, or perhaps spells its true name. Only one individual in all the multiverse knows the word for which this glyph stands, and to speak it is to grasp the fate of the City of Doors. This is The Lady of Pain.