[Cainehill]

As Arryn said - it'd help if you mentioned the nation, etc; also the map you've been playing (some maps cause different strategies than others, ie Cradle of Dominion & the MiddleEarth map among others have chokepoints on the map), who got behind your lines - Caelum, Pangaea, Vanheim especially can be frustrating that way.

Nations: Jotunheim and to a slightly lesser extent Abysia (possibly others such as Ulm) are well served by putting 10 PD or so in every province, as it'll stop many small invading armies. Even better, 10 giants or Abysians are great when beefing up your responding force of mages/missile troops/whatever.

Build castles strategically; also, you can build PD strategically, as your opponent is much less likely to run into the province that his reports show as heavily defended. Then, move your response forces into the provinces where you've channeled him, trying to anticipate his movement with a force large enough to destroy him - or at least to kill some commanders.

Assassinating his commanders also helps - nail a commander that was leading troops, those troops get left behind when he moves. Likewise certain spells - Fires from Afar is a relatively easy one to research / use - will kill some of his troops, and possibly some of his commanders, again leaving some troops stranded behind.

But really, the best thing to do is keep them from getting behind your lines. Difficult against Caelum and the others I've mentioned, but.... Or, ignore his raiding army, and ransack _his_ provinces instead, while trying to attrit his raiders.

And throw up PD, armies, or fortifactions in key provinces - ones with a good independent to be recruited (sages, jade amazons, longbows, etc), or with great gem or gold income or resources. You don't care too much if someone takes a province that gave 2 Gold a turn and 3 resources; not like when you lose one that gives 75 gold, or 6 gems a turn.

If you have crap PD - try putting up a screen of crap PD, but also leave recruited, or better, summoned, troops there that are half decent. (If the summoned troops are magic, you need to leave a mage as well, unless you get a mage as a PD commander, or the magic troops start going poof. But many summoned, magic, troops, aren't considered 'magic', which is surprising for some of them. Wolves okay, but vine men aren't magic either, which is odd.)

HTH