Well by the PCs anyway. Essentially, I placed a portal to Sigil in my campaign. I hadn't expected them to find it quite yet, but what do you know? They went down to the second level, and went through the one exit in the first room that led to a teleporter, and now there is a strong possibility that they will find the portal to Sigil if they pick the right way twice...
So I need to detail what might happen if they take the portal. Where exactly in Sigil might it lead? What kind of challenges will the party face as they try to adapt to this new environment? I need to know by perhaps as early as Wednesday.
Drop them into the middle of the Great Market. Emphasize the weirdness in the usual ways: An angel and a demon sitting at a table, sharing tea; the press of a crowd filled with races that they're unfamiliar with; merchants hawking impossible goods.
If you want to give the party an easy out, an honest tout could suggest: Hey, Clueless. You better nip back through that gate there, unless you want to get lost here in the Cage."