It's all a matter of Levels. You only defend levels 1, 3, 5 and 7 (The Citadel). The abandoned levels will cause confusion among the zombies and act as firebreaks. Once they over-run a level and find an empty one, they will initially think that is all there is the party is over. Eventually enough will meander aimlessly over to the next inhabited level, but not before they have experienced a psychological effect known as False Party-is-Over Syndrome, leaving them unmotivated and suspicious of the zombie management.
And Gates. You fill in the gates with rubble and rocks, leaving only one small opening... just large enough for a zombie to poke his head through. But don't use every rock. Leave one rock loose and handy.
Lastly I would use a Deception and Misinformation Campaign. I would put travel posters up on all the walls and buildings, but cleverly have crossed out the original location names and instead written 'Mordor' on them. All over the Pelennor I would put up direction signs on poles with "That way to Mordor", etc on them.