Arcane Lock is a working of magic laid upon a door or gate to make it fast against all who are not welcome — a barrier that turns away the unauthorised hand and keeps a chosen threshold secure. The scroll bearing it was recovered from a spellbook, and so came ready to the party's hand for the shoring up of their defences.
In the assault upon Yedikule Fortress, the attackers under Hamit the Quartz of Damascus came against the walls by night, electing to force the main gate and the ground-level entrance. There they met the several traps and contrivances the party had set, the Arcane Lock amongst them, which sorely complicated their entry. The spell was one more of the vexations visited upon Hamit and his crew as they laboured to reach their quarry within the fortress.