[Chazar]

I usually go for either 1, 6, 11 or 24.

I put 24 in chokepoints that I do not want to cover with a fortress for whatever reason. The PD shall search for sneakers as well as being an upkeep free defence of the chokepoint, which works if there is some back up by mages and very few quality troops. Therefore I buy a lab as well, so that the mages double as researchers and defenders. I think that 21 is a waste, since the 22 gold for one point yields two units. So I stop around 24, i.e. 12 gold per unit obtained. Chokepoints defended in that way wont deflect serious attacks, but force the enemy to bring much more than a call of winds and a few sneaking troops.

I put 11 if I expect reoccuring heavy battles in that province, so that I have some freely regenerating chaff.

I always put 6 to protect myself from scout attacks. I think 6 is better than 5 because it yields a better scout report to the enemy, who cannot distinguish 6 from 11 PD.

I only put 1 if I am broke or if the province will be lost immediately (e.g. province was won by flyers or long-range summoning).