There is no one answer to that - it's up to you to figure the right balance for each situation. Sometimes you can invest in PD and it just gets wiped out; other times (especially if you combine it with a force of troops set up to help it out) it can turn the tide of several battles, causing many enemy losses and sparing your own forces.
Adding to the impossiblility of deciding is the random luck-and-order-scale-influenced chance that the local leader will randomly organize additional PD on top of what you invested in, and since it is an increasing scale, PD 25 +10 is worth far more than PD zero +10.
If there were a simple or an always-right answer, it wouldn't be interesting.
PvK
Re: Province Defence?
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