OOC:
Silvanus' spider form's blindsight will definitely help with the fog (out to ten feet, anyway) and will also "see" through simple illusions, so you should have no trouble navigating or finding glamoured doors. The landings are barely large enough for three humanoids to stand together, certainly much too small for a mammoth. :)
The webs, even when burned away, return very quickly - while you've been talking to Peregrine, the ones on the stairs you just came up have regenerated. You can't tell where you've been, let alone whether anyone else has come through.
Avaris, let's say you have one box of paper birds on you, with 1d6+3 sheets. You can get more whenever you have access to the rest of your assets.
As Avaris releases the paper bird, it flies up to the ceiling but then dramatically flares into ashes.
"No," Peregrine shakes his head sadly.
"We are still on our native plane. Unless he has travelled to another plane himself - which is possible, though I can't think why he would - then that is not a good sign."
The three of you continue to explore the tower. You don't find any hidden doors, but the magical effects are definitely beginning to play with your minds - you get turned around, walking up stairs when you thought you were going down, even finding yourselves arguing with each other about which direction
is down. Eventually, after what feels like a dozen flights of stairs, you reach the end of the staircase.
An imposing door, seemingly fashioned from mithril, waits at the end of the landing. Silvanus stops as the bricks in the wall next to him start to move, grinding together and parting to reveal something like a mouth formed of stone. It speaks in a deep, gravelly voice.
"INTRUDERS. I AM IMPRESSED THAT YOU HAVE MADE IT THIS FAR, BUT NOW PLAY TIME IS OVER. GO NO FURTHER. THIS IS YOUR FINAL WARNING."
Having delivered its message, the wall returns to normal.
