(Mostly) fake minefields, with enough real ones and lots of booby-trapped fakes to slow them down.
If you have a good idea about the way the Warlord's forces would be coming, set up some tempting almost-hidden caches along their line of approach with whiskey laced with antihistamines, which will have them falling asleep on their feet or behind the wheel. If you're feeling evil, rig up some Gatorade bottles with antifreeze--their livers will thank you for it.
Otherwise, scorched earth
Contaminate water sources--wells, reservoirs, apparently sealed bottled water--with some easily-obtained E. coli bacteria (I'm sure you can figure that one out
).
Stealth raid to steal or damage the breech blocks of the artillery. And the good old tried and true "sugar in the gas tank" trick.
If they reach the town, make sure they will be channeled into dead-ends and crossfires,with rat-holes allowing the defenders to shoot, scoot, and shoot again.