[Baalz]

Well, the problem is I suspect much of the advice you'll receive about defending in SP it going to revolve around exploiting the AI's weaknesses. Once you know how to do this the AI rapidly become not too challenging because it can't adapt, so I'm not gonna give you any of that type of advice (other than to mention the AI is pretty superficial in deciding to declare war, and never un-declares war once it's started). Here's a couple pointers on general defensive strategy though that are universally applicable.

1) PD effectiveness varies a lot by the nation that you're playing, but is never going to be strong enough by itself to stop any real army. PD is generally useful for stopping light raiding and adding some support to your regular army (when it's defending). Dominions is designed to be a dynamic, offensive oriented game, not a defensive turtling type so offense is usually going to have the edge, which leads us to...

2) Offense is the best defense. It doesn't matter how many of your provinces the enemy can take if you can take two of theirs for every one they take from you. If your enemy is spreading out too much he's vulnerable to you dividing and conquering his armies, if he concentrates too much he's vulnerable to you overrunning him.

3) Depending on what nation you're playing and what units you have available often a small force (usual a mage with the right script) will greatly augment your PD. A cheap mage casting protection, body ethereal, raise dead, etc. can amplify the toughness of your PD considerably, and may be worthwhile to spread around if you're getting attacked on multiple fronts by not particularly strong armies.

4) Fortresses won't kill any invaders, but they are significant speed bumps. Don't underestimate their defensive value.