Clausewitz Study Guide.
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Here's useful quote for someone defending their turf:
Not only reason but experience, in hundreds and thousands of instances, show that a well-traced, sufficiently manned, and well-defended entrenchment is, as a rule, to be looked upon as an impregnable point, and is also so regarded by the attack. Starting from this point of the efficiency of a single entrenchment, we argue that there can be no doubt as to the attack of an entrenched camp being a most difficult undertaking, and one in which generally it will be impossible for the assailant to succeed.