Redcloak's Escape (SOD Spoilers)

[The Giant]

I always assumed the cave was warded against (at least up to midlevel) teleportations, plane shifts, and stoneshapes.

Lirian was epic, that trap down there was her "big project" to keep all kinds of spellcasting enemies trapped... so why not just "magically ward" it with spells from the book or a homebrew-version (she might even have help from Dorukan) and be done with it?

This. The implicit assumption of several lines of Redcloak's dialogue is that he already tried everything he knows how to do. I just didn't show it all because I had limited space, and because I assumed that the reader would understand that if a smart, tactically-minded caster like Redcloak says, "X is the only way out," then he probably tried the easy ways first.

After all, Lirian would have needed to ward her prison against people coming in to rescue them, too, which means locking it down to plane shifts and stone shapes, even if she thinks her prisoners are incapable of them. A restored Xykon, however, is more powerful than Lirian could ward against.