Probably, just my theory, of course, the seal WAS an entryway into the, or at least the local, hellmouth, but what was directly beyond it was a sort of antechamber, guarded by the army of ubervamps, placed there to block entrance by anyone or anyTHING from this side. It could have been set up by the Old Ones when they were at war with the humans, and, after their defeat, was sealed in order that the remnants of the Old Ones' army would be contained, sort of what the Initiative did with their facility after Adam was defeated, only on an obviously much larger scale.