[sector24]

The lowest levels of PD are far more efficient than hiring troops. This is assuming that you expect to get attacked imminently, and are not just building them for the patrol or extra commander.

I'm playing as Atlantis EA, so let's use an indy province with Atlantean Spearmen as an example. These guys are 8 gold and 2 resources. Everyone puts 1 PD in their province to avoid getting taken by a scout. After that, think about what the best use of your gold is. For 2 gold, you can get two units, or you can spend 8 gold on that one spearman. So obviously, you're getting a better deal for your gold by bringing your PD up to 15. At 16, presumably you could buy two Atlantean Spearmen for the same price as the 2 guys you get from PD, so it depends on the stats of the units.

That's the basic idea I use to determine a good level of PD. I compare the gold spent on the local units to the quality and number of units I would get from PD. It definitely matters what the local units are and what nation you are playing to determine where PD becomes less useful than recruiting units.

Of course this changes if you expect to pick them with a commander somewhere down the road. But there's no advantage to having 5-15 units sitting in one of your provinces eating your food month after month.