The defense of the City of Greyhawk is a bit of a question mark.
So the first defense is the New City Wall. This has been tested by a few bandits and “bands of ruffians” but has never encountered any serious efforts to overcome it. This is coupled by the Old City Wall which serves as the fall back position for the city. And if it were to fall the last stand will be in the Citadel.
All that is well and good but you need manpower to oppose the city’s assailants.
The text states:
. . . The standing garrison of the City Watch, a complement of some 800 men, is a steadfast and capable force, well trained in combat. It fights as two brigades of three companies each. One company fights with spear, another with long sword and shield, and a third with long bow and short sword . . .”Niles, 4
800 men seems like a woefully understaffed City Watch for basic peacekeeping efforts, let alone for the defense of a city which houses nearly 58,000 people within its walls and well over 75,000  that might flee there in the event of an invasion. However, when we compare it to modern police force standards it turns out to be an incredibly generous number of individuals allocated for this task.
For its defense the City can call up 5-8 companies of mercenaries, each numbering between 100 and 200 members in its moment of need. These companies represent well trained, veteran fighters with various setups for gear. And if things get really bad every citizen in the city – every wizard, cleric, and adventurer – is expected to man the walls if the City is attacked by a significant threat.
As far as our purposes go there are a lot of opportunities to establish a better organizational structure for the City Watch and to establish who the City will call on when it needs to going forward. We’ll be doing that after we gain a little bit more knowledge on the population of the city. So look forward to that.
 It’s an odd turn of the phrase that is used here and perhaps that is an issue brought on by colloquialism. I’m from the South Eastern United States and while we have countless “Old Cities” dotted about you never – or at least I have never – encounter the phrase “New City.” New areas of town take on names of streets, or famous events and landmarks.
 the separation within the text between bandits and “bands of ruffians” seems an odd distinction. What constitutes a band of ruffians? People who just feel like assaulting walls of relatively well protected cities?
 Source the Greyhawk Boxed Set pg 23.
 As an example, my home town has a population of approximately 11,000 and only has 34 police officers; or an average of 1 police officer for every 323 people. The City of Greyhawk, by comparison, averages one City Watch officer for every 73 people.
Niles, Douglas. Book 1: Greyhawk: Gem of the Flanaess A gazetteer of the Free City of Greyhawk and the Surrounding Area. TSR, inc. 1989. pg 4