The stone spider was originally intended to serve much the same function as the vault, insofar as it was intended to house civilians in the case of a siege. However, the fact that it was open to the outside caused security problems that resulted in it being sealed off.
Shortly after it's creation, it was realized that the spider fortress was a major security problem, as it provided a virtually unguarded access point for invading goblins. The surface levels were sealed, a floor placed over the stairwell into it's lower levels.
Later, in the year 666, Sir Brenzen ordered the complete sealing of the spider Fortress. In the tunnel leading to the spider fort, the meeting area of the Demongate Guardians was dug out by the aforementioned.