It's a complex question really, but the short answer is by force projection - sending units out into the surrounding countryside and dominating that so that an enemy can't get close, or at least can't get close without being detected at which time a suitable response can be sent by the unit commander.
If an enemy gets to within firing distance of the town, the defenders have failed to "protect" it and from then on can only use it as a defensive position (and hopefully drive the attackers away). Damage to the town and it's inhabitants is at that point virtually unavoidable.
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If it moves, shoot it, if not push it, if it still doesn't move, use explosives.
Nothing happens in isolation - it's called "the butterfly effect"
Mors ante pudorem
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