One-way Walls

Makes walking around dwarfhomes in the Myrskyt Mountains a confusing prospect, that's for sure. "oh, I'll just retrace my steps" you think, and then you turn around, go to take a corner and WHAM! stone wall, right to the nose.

— Jasper Cameron, Leader of The Bronze Ravens.

It is truly a marvel of dwarven magic and engineering. When traveling in one direction your view is unobstructed and you can pass freely, yet from the other direction you are confronted with a wall of solid stone. No magical token or spell is needed, and woe be to the person who tries to get through using a passwall spell or some other magical means.
After all, the walls were originally designed as a defensive measure against elementals and other threats that live in the underground depths, to whom regular stone is no barrier at all. The old entrances to Myrskyt dwarfhomes are labyrinthine in nature, designed so that defenders could rapidly deploy to where they are needed, and shoot and harass any threats from near absolute safety. Defenders would have the way back through the maze memorized, able to retreat in the unlikely event that the wall showed signs of being breached while attackers were confronted with another corridor in a maze, unsure of where the next spear or bolt may come from.
The only notable drawback in the design is that should a defender be gripping a spear to tightly it is possible for them to get pulled through the wall, at which point their survival depends on their ferocity and their speed. Because of their effectiveness the secret is so closely guarded that outside of the forts around The Emerald Hills no wall has been built on the surface, and any unauthorized study is punishable by death.

The Myrskyt dwarves are, according to most texts, the originators of the first passwall spells and as such undoubtedly accounted for it in their later magical projects. Passwall, as most will know, is a spell that manipulates space to create a temporary passage from one side of a wall to the other while not interfering with the structural soundness of the surrounding material. Now, dwarven stonework is already resistant to manipulation by magical means, but in the case of one-way walls we come to a rather peculiar issue. For a passwall spell to work one can think of it as needing an "anchor" on both sides of the wall. Yet a one-way wall, once constructed, only has 1 physical side. If you are lucky, your spell will fizzle out and nothing will happen when the second "anchor" is not found. If you are unlucky, your spell will find an anchor on some other plane. If you are really unlucky your passwall spell will loop back on itself creating an astral rift sucking up you, anyone near you, and a majority of the surroundings (a good bit of which are probably structural supports for the tunnel you are in) and depositing you somewhere in the infinite expanse of the astral plane.