Defending Provinces.

[The Panther]

In SP, if you put 10 PD everywhere, the AI will be afraid to attack, for they can see your PD value in every province and grossly overrate it. If you put 20 everywhere, they will leave you alone. This is a cheesy way to beat the AI easily by attacking them on your own terms.

But don't even remotely think of trying that in MP, as it is a total waste of gold. The uses for PD are:

1 PD - use everywhere so you can see your opponents troops if they attack.
3 PD - stops the black hawks for most races.
5 PD - can also stop the wolves.
10 PD - can catch scouts and spies and will stop many more of the annoying spells like arouse hunger.

You can even put up a big bunch of PD in a single province if you believe an enemy will attack there and you need some fodder for your own army to meet them in battle. It is a quick and cheap way to get a bunch of wimpy troops instantly for a key battle.

As for your capitol, that free 25 PD will help you a lot if you can somehow catch an enemy trying to seige it and move in your own army or patrol on the same turn.

But PD does need to be fixed somehow to make it far more useful. Like adding a mage for each 5 PD. Or maybe something else too.