Province Defence?

[Nick_K]

Isn't it 20 (21?) that gives the second commander?
In SP I like to use lots of PD because it's less hassle - no micro with province defence. PD of 20-30 helps discourage AI attacks on border provinces, and PD of 50+ backed up by an army allows bottlenecks to be defended against large armies with few casualties

[Loren]

PD works wonders against the AI.

I generally put 20 everywhere to keep from losing provinces to spells and almost all random events.

Beyond that, chokepoints are a wonderful place to use it. I've put 125 in a choke point and quite a few troops besides--and an incredible number of AI troops died against that barrier. I was killing over 100 units a turn there for years.

[Gandalf Parker]

I think the 20-something number was for catching sneaking units like scouts, spies and assassins

[Endoperez]

PD of 10 or more helps catching stealthy units and armies (pg 67 of the manual). At PD, the nation gets another type of commander and starts getting defenders of another type (pg 97).