A hardcore move would have been to block all exits to normal sewers from all the Warrens levels, since the ledgers explicitly state that those sections were bricked off. It doesn't make any sense then to have any of them be separated from the normal sewers by doors, like in Warrens 1, 2 and 3. It would be a major hazard for the Nosferatu. All Warrens haters would of course object to that change :D
Now that's a horror scenario for the ages.
Is it, or didn't you prepare for your trek properly? C&C my friend.