[Sorry, my map differs from the one on the wizards site, slightly. 3B refers to the wooden planks on the east side of the building. 3g refers to the open spot of water (the rip in the building) on the west side of the building.]
The boardwalk where the group is now standing is vacant, and two wooden stairways lead up into what were once windows of the town hall. Stairways leed up, onto the rooftop, flanking it on the north and south side. To the south, a bridge leads into a building in serious disrepair that looks to have been part of the larger overall town hall at one time. A simple wooden door locks that path.
(So to simplify it a bit, there's two stairways both leading up to the top, and a doorway to the south.)