So in my RPG campaign, my players are sneaking into a keep occupied by vampires. They were told of a secret tunnel that leads from the cliffside and into the keep itself. This tunnel would've originally been an escape route for the noble family that used to live there but has since been abandoned.

Why would it be abandoned? Preferably something monstrous or dangerous, but this tunnel would have been in use for hundreds of years until less than 2 decades ago.