I have not read every reply, just skimmed, so I apologize if I missed something.

in one of your earlier posts, you mentioned the lower entrance is opened up and beasties are let loose. If that's the case, from a story perspective, I would suggest two things:

1) make the entrance MUCH smaller, say around 2-4 feet wide at most( it appears around 15-20 feet wide, and spread some rubble around at the base, including a few very large boulders, possibly having a slope down away from the entrance and toward the town. Its a bit hard to tell due to the darkness of the map overall, but you might already have the slope(it kind of appears that it might).

2) have above said opening created by a recent earthquake or some such, creating the newly found opening.

it's clear that the town is lower than the entrance to the cave, just not by how much. I would also suggest making the lower cave much higher up from the top entrance and ruins to make it harder to access(such as say a cliff face south of the ruins

Alternately, you could have any other event create/free the opening, but it should be something that recently happened within the past few days. I know in most cases, if a new cave opened up a few hundred feet from a town, there would be something to go in there in the first few days(possibly to disappear spurning the wandering adventuring party upon the town the investigate just in the nick of time)

Is the new entrance something that's known in town as being covered over(perhaps people in the town buried it at some point in the past and forgot about WHY?) or are they completely unaware its there?

There's loads more questions, but to not just throw out loads of things, it really needs the beginnings of a hook.