Let the dice roll what they may
Give them a chance to survive: have them all roll spot checks & tell the one to roll highest that there is a partially concealed, rusted steel grill on one of the walls. If the PCs work together, let them break through the grill to find a concealed stairway/ladder leading up to one of the city alleys where they can run away.
If some or all of the PCs die, so be it. Let the players make up new characters (IMC it'd be at the same level their old PCs would have been if raised from the dead) recruited in the same city to investigate the wererats that killed their previous PCs.
This way you don't have to scrap your entire campaign plan & the players get to know that you won't save them from their mistakes, which can give you a real edge if you try foreshadowing big/dangerous encounters in the future.
