[Endoperez]

Currently, the best you can do is recruit some provincial defense and hope that it keeps the AI at bay... If the AI thinks that you have strong defense, it doesn't attack you in the first place. Sometimes you can also use strong independent provinces as an upkeep-free guard. 80-strong army of Knights and Longbowmen will keep AI armies from coming to you through that province, and while the income might be nice the fight would be costly!

However, buying more than 11 is rarely worth it, but 11 allows PD to occassionally catch enemy scouts, and with nations like Ulm, Marignon or Jotunheim might even help if enemy does attack. For any real threat, PD alone won't work, but recruiting some independent units in addition to PD works much better. Some heavy infantry with Marignon Crossbowmen supporting them works very well as long as the enemy army isn't too big.

There are no more buildings in DomII, and I haven't heard about any more in DomIII. I'm not sure if PD will be changed, but I seem to recall one of the developers stating that making defense a viable option would make the games boring. Currently, some people already consider gem-generating items like Clam of Pearls or Earth Blood Stone a viable strategy that really pays off after 50 or so turns... Games can last so long that it isn't fun anymore.