the Sibling's Hidden Tunnels
also known as hidden corridors of Urithiru
Sealed passages built into the upper floors of the tower behind gemstone-locked doors, each door a slab that splits silently down its centre when its embedded gem is infused with a small breath of Stormlight. The Sibling uses the garnet veins to lead Kaladin from gem to gem, through corridors and a hidden stairwell, into a chamber on the eleventh floor the enemy has not searched. SA:RoW:Ch 44