First off you should decide what goals do the ghouls have beside the standard kill and eat everyone. Are they looking for an artifact, here to kill someone(s) in particular, or using the confusion to slip a spy into the city? Whatever those goals my be, they should be one of the two major determinations for where combat will be concentrated. The second is where the defenders are able hold off their progress within the city. Unless you concentrate all the ghouls into one location, which would be rather boring (not to mention a lot easier to destroy), the players should encounter several the following potential situations while moving through the city:
1} Ghouls vs Guardsmen (of which there will be 3 types: evenly matched, ghouls winning, and guardsmen winning.)
2} Ghouls vs Civilians (this includes mid to high lvl NPCs; I would put the win vs loss at 75% in ghouls favor, 20% evenly matched and 5% in civilian favor)
3} Panicked Civilians/Mob fleeing. (Risk of being trampled, separated, etc)
4} Areas devoid of activity and so no signs of combat. (These can be used not only as a breather for the players, but if placed near other areas of combat, can add weight to the struggle between the two forces. Maybe its clear here because the guardsmen are not willing to allow a single ghoul past [Guardsmen taking extra risks to stop ghouls thus causing them to be losing].)
5} Areas devoid of activity, but clear signs of combat. (Most of these should depict slaughtered civilians. Though a couple should have a large group of ghouls with a few guardsmen and/or mid to powerful NPCs)
6} Ambushes (This can be from the ghouls, nervous guardsmen who didn't take time to check who it was before attacking, or even a group of enemies the players might have made within the city.)
7} Looters/Scavengers (Depending on how well known [or obviously powerful] the players are should determine how likely these two groups will attack the players.)
8} The lone Ghoul. (One or two wandering around in an area should make the players wary and it can either call or run for help.)
9} Roving group of Ghouls/Guardsmen (Both should have sustain damage from whatever fighting they have just recently won and are on to the next task. Ghouls can either be dealt with or hidden from. Guardsmen can be directed to help trouble locations, give important information on how nearby parts of the city are fairing and/or travel with the player to assist however they can.)
Honestly, if you don't have a map of the city, I would make one or find something something decent online to depict one (doesn't have to be the city). On a spare copy, mark down the locations where the above situations will be when the players are going to arrive. Set up a basic flowchart to show the course the battle across this city as if the players did not interfere. Determine a general rule for conflict between groups, IE. Ghoul groups kill half of a Civilian Mob in 5 rounds, etc. While this flowchart will be shot to
once the players start interacting with the various encounters, if done right you should still have a general feel for how the city fairs until it done.
Best of luck and hope you enjoy.
