[Saxon]

I remember reading that 11 and 21 were key cut off points for PD. At 11 you can catch spies/scouts sometimes and at 21 the chances are quite a bit better. I also thought that 21 was where you got the extra commander, not 20, but I could be wrong.

However, look at the cost of PD over 11 carefully, as you do not get good value for your money on a per unit basis. If your PD is 14, you pay 15 gold for the next step. Getting another cheap unit that is stuck in the province may not be the best use of your money. Like everything else in the game, you have to balance things.

Your question about the best item is not easy to answer, as it all depends on what you want to do! Think of them as tools and then figure out what you want the tool to do. I know this sounds like an answer from a yogi, but it is true.