Again if your HL demographics allow your kingdom (and/or the opposition) to muster 2 or 3 groups of HL characters, don't bother with the army until you have neutralized the opposition's HL forces first.
I see a lot of people think that just because the defenders have the same number of HLers as the attackers, it all comes out in a wash. In practice however that couldn't be further from the truth...
IOCs, we've run many of these kinds of battles over the years, what you find is that you require many, many more HLers to defend
anything than it does to attack a large target like a city or a army for example. And these HLers will have to stick together in groups for the most part or risk becoming easy targets themselves. The attacker has all the advantages - he can attack where he wants, when he wants and in whatever manner he wants. Defensive magic in D&D is way too limited in ability and area to be able to counter these kinds of attacks. With a little stealth, a little coordination and a little intelligence you can make devastating hit and run attacks with little to no fear of reprisal. See my previous example for ideas.
Marching an army and laying seige to your enemy is a cool notion, but the way D&D is geared, particulary with teleportation, flight, invis., endless variety of attack spells, etc. an army is a huge, slow moving, nigh impossible to defend target. An army is simply too weak and too vulnerable unless there are very, very few HLers in the world. And if you're marching an army, you'll require an inordinate number of HLers just to provide even the most meager defense, leaving your cities even more defenseless against the kinds of city attacks I mentioned in an earlier post. The HLers defending the army would have to scramble back to their cities to try and defend them (leaving the army vulnerable) or leave the cities to burn while you're on the march, It's a lose-lose situation.
Win the war first, then bring in the army.
In my new campaign I've been very careful about the levels of magic, monsters, and in particular, a few of the powers that can really be civilization breaking. There is no teleportation, spell-based planeshifting (with the exception of a very few Druid-built gateways), or spell-based flight (but a flying mount is possible). Scrying-style divinations can only be done through a kind of ritualized astral projection (which has dangers of it's own) and there are no greater than 5th-6th level spells period. There is no spell-based polymorphing/shapeshifting, but there are a couple of supernatural PrCs that allow for some limited shapeshifting. That said, you can astrally project through a spell and "possess" an animal which I think is kinda neat. If you're powerful enough you can try and possess another person, which essentially replaces domination in the game. That probably gives you a good enough idea of how I'm weeding out the problem children...
Cheers,
A'koss.