[@Who is Who] I was thinking since the main problem for the pedestrians is that they must travel further each time to get anything (I assumed that they go away for resources and food, right?) We could add a big ruined city plagued with zombies but also with enough resources to keep Citadel functioning for over two years. 
It could be located far from the ruins of Rivergate City. So pedestrians would have to manage- not only to get across the broken Bridge but also manage to cross Rivergate City safely. 

I'll let you guys choose the City's name.