Don't Move Me Cheese... or....

[Taqwus]

It really, really depends. You can't afford to mass armies everywhere, unless you're Dead Ermor or the game's already over, and if your opponents have decent long-range summons/attack spells (Horrors, Ghost Riders, Earth Attack, Mind Hunt, Wind Ride come to mind) the odds of stopping cold a reasonably focused attack via just a standing defence are not too good. Likewise, flyers and remote attack spells make a mockery of border-only defenses.
It's good to have a reaction force of some kind if you can manage one, and to have your own spells to hit back during the magic phase before an incursion can move on. In addition, hiring mercenaries in the vicinity can provide some immediate local reinforcements.

[Spacepain]

So, generally more reactive actions are emphasized over preventive measures.

[st.patrik]

Certain nations I always buy defense for, in almost every province. Ulm, for example, gets those arbalests, which is a great backup force to a standing army, and is pretty good on its own, since sometimes you can kill enough the first volley to rout the attacking force (I'm thinking CoW/W here).

Other nations I pretty much don't spend much on PD unless I am expecting a big battle and want some fodder - in which case I'll buy some PD and then put my real army on the flanks. Hopefully the PD will soak up most of the damage (which is all they're good for) and my main force will survive mostly intact.

The other use for PD is to buy a couple of points just so that if you get attacked you'll know exactly what hit you. So I quite often do this, since a point or two costs hardly anything.

That's in MP of course - in SP the AI is too stupid to realise that your 21 PD is a pushover, since it's composed of militia, so it ends up being an effective deterent (for a while).

[Norfleet]

Originally posted by Spacepain:
So, generally more reactive actions are emphasized over preventive measures.

Preventative measures can work: Constructing defensive fortresses can slow down the progress of any invasion, and nullify Ghost Riders entirely, as they do not stick around to siege the province. Preemptively pillaging the entire province will reduce the income loss of losing the province as well.

Standing defenses, however, are rarely effective against a human player intent on raiding. PD *IS*, however, effective against the AI, who tends to throw Call of the Winds and other such spells around fairly randomly, and doesn't really try harder if he fails: This CAN be stopped by PD.

As a general rule, PD is more useful vs. AIs than humans.

[Norfleet]

Originally posted by Spacepain:
So, generally more reactive actions are emphasized over preventive measures.

Preventative measures can work: Constructing defensive fortresses can slow down the progress of any invasion, and nullify Ghost Riders entirely, as they do not stick around to siege the province. Preemptively pillaging the entire province will reduce the income loss of losing the province as well.

Standing defenses, however, are rarely effective against a human player intent on raiding. PD *IS*, however, effective against the AI, who tends to throw Call of the Winds and other such spells around fairly randomly, and doesn't really try harder if he fails: This CAN be stopped by PD.

As a general rule, PD is more useful vs. AIs than humans.

[Zurai]

Depends entirely on things mentioned earlier, like what race you're playing and what size map, etc. 20 province defense as Jotunheim is about 50x more effective than 20 province defense as Machaka or C'tis. I rarely even bother with PD as those two nations because their militia has LOUSY morale and tends to rout in entirety at the drop of a hat. 20 PD as Jotuns is effective enough to stave off small to medium sized armies and most distance attack spells until the very late game.