I am DMing a game with an evil party who wants to tak over the world (bwa-ha-ha-ha and all that). I'm trying to come up with some idea what challenges they and their allies would have to overcome in order to take over individual enemy settlements.
One challenge would be to kill or drive off the high-level residents of the settlement, but of course the question then is how many of them are there, what are their classes/levels/etc...
Also I am wondering if there is some way that the lower level inhabitants could be mobilized to drive off attaks or raids by a party of mid-level (party average 11) PCs.
Depends upon how large the settlement actually is, and how much warning they are given. As a standing force (guards, soldier and Noblemans Levies) you would not expect any more than 10% of the population. Most of these will be fairly low level characters and are not likely to pose much of a threat to your evil characters. Any more than this and the drain on resources for the town would become too onerous to support for any period of time. During a seige situation you would temporarily have a lot of retired and inexperienced people joining up to help defend the settlement. Say 30-40% of the population max. Many of these individuals however will be first level with little or no equipment. So the question becomes do the evil characters give the settlement time to organise an appropriate defence? Otherwise, only the standing defence force will be available to fight them...until the resistance movement starts (Viva le Resistance)
If they give the settlement time to get organized even extremely low level characters have the ability to do serious damage in terms of mundane traps for the unwary attackers, EG pit traps, area effects like molotov cocktails thrown from the rooftops, and poisoning the food.
If they had enough warning they could also fire the crops to deny the attacking army the forage, the old "An army walks on it stomach" defence. Your evil characters' allies are not going to want to fight if they are starving and will probably start to desert if the combat becomes protracted. Low level characters because they know they are outclassed will fight extremely dirty so expect them to do things like stampede cattle through the streets, target the evil guys horses and wagons, rig houses to fall into the street, coat all roads with hastily made caltrops and marbles and things like that.
Hope this helps