It’s something that sounds cool in story, but the limitations of mobs makes it less interesting in practice. There’s a couple plugins that try to do things like this with only marginal success using pretty hackish methods that aren’t really ‘gameable.’
How would you make this fun? Just spawn zombies randomly around a city and hope they make things interesting instead of just forcing people indoors? I could turn hard mode on so they could break down doors, but then people would die of starvation randomly. If I used ghasts and creepers, then we’d actually have something to fight for, but that also means real consequences…. and it’d still be better to run or hide.
I have a couple ideas on how to make some mob battles interesting, but I’ve never cracked this ‘defend the city’ game mode.