Then he may have some problems with this.

Kindly explain.

The War Without Rivals ('87-'93) was fought between Count Aericnein Neska of Barstoi and the unlead and chaotic nobles of Count Olomon Venacdalia of Ardeal over the question of whether Ardeal was mismanaged, or criminally mismanaged. Threats of foreign intervention (that is, from the Prince  eventually persuaded Neska to withdraw, burning the land behind him, which produced the infamous Furrows.

Threats of foreign intervention did this because the entire thing had been bogged down into an endless train of sieges and Chevauchée raids, in which the purpose was to devastate the country. Neither accomplished anything, and by the end of it a long, long line of fortresses stretched down the entire Ardeal-Barstoi border, the Ardeal ones little more than towers intended to hold off wizards and trebuchets, but the Barstoi fortresses were built with the full prosperity of the most flourishing county in Ustalav and were full-scale castles, manned by knights and well supplied and equipped, from which long-range fire could devastate any attackers.

We got them while you were messing around in Ardis.

... Yes, Wielki Ksiaze's general does have a problem.

If the army leaves behind troops to invest every castle on both sides of the border, it will be exceptionally vulnerable both to the Hellknights of Barstoi winning a battle, and, worse, to them riding around the army with their cavalry and relieving the castles.

And if the army settles down to besiege fortresses, the Hellknights have freedom of movement to do whatever they like.

Fortunately, says Ksiaze over his permanent Telepathic Bond, this is a very temporary problem. Our triumphs in the west have shielded us against Lastwall, and soon we shall control the north as well. More reinforcements will arrive shortly; invest what fortresses you safely can, maintain surveillance of the enemy, and prevent them from carrying out any attack that might delay our inevitable triumph.

And how commences his campaign against Odranto? There is a great host of his champions marching north, with the better part of his cavalry and many dreadful specters and powerful necromancers, under one of his own Lich-lieutenants, Kasimir Konor, a wizard of considerable (if, well, less) repute.

New Razmiran would be much, much happier if it had a week to train new troops, serious, what is this "marching all day and night straight past castles which you have the strength to besiege and bypass simultaneously because you don't care about supply lines" nonsense, but with every baronial levy they can pull together, and the support of the Regent of Odranto, New Razmiran has nonetheless assembled about forty-four hundred heavy infantry, mostly their Razmiran veterans, in the usual eight-to-two pike-to-shot (heavy crossbows or longbows) mixture with every cleric of Pharasma in the county drafted to stiffen it with healing, bolstered by their usual outsiders and Razmiran Priests, as well as two thousand militia troops (archers and slingers) that can be expected to rout the first time things go badly, eight hundred aristocratic heavy horse and four hundred light horse scouts and skirmishers, and with this army they intend to hold the ford of Berus against any advance.

Unfortunately, this force has the usual fighting-undead problem - pikes and bows are both effective at finding small vulnerabilities in armor and punching through, when facing armored enemy soldiers. And, really, anything that punches a hole is effective when up against sacks of meat and blood that don't have armor.

They are... not... effective against zombies and skeletons.

The lords of Odranto prepared for this! Their heavy infantry (what they have of it) carries large polearms with an axe-blade on one side (for zombies) and a hammer on the other (for skeletons) and a spike in the middle (for cavalry), which don't have quite the reach of pikes but have enough reach for most purposes. And then they go to war with maces or warhammers as well as (or instead of) swords, for their backup weapons.

The soldiers of New Razmiran... did not. All they have of these fine weapons is that they were fortunate enough to get the lords to provide them. Most of their troops are just going into battle with the usual pikes.

New Razmiran really, really wanted time to rearm, retrain, and get ready for war.

Yeah, that's why they aren't getting it.

The fords of Berus, you say? Not a bad name for a battle. When Kasimir gives him his sending, Wielki Ksiaze will Teleport in in person with his personal reinforcements. He has a zombie griffon and a huge evil longbow and a bunch of big scary bodyguards and quite a lot of low-level spells he's not doing anything else with today and no intention whatsoever of getting closer than twelve hundred feet from anything that might be a massed formation of archers in disguise.

And, of course, he has his own army! Nine thousand skeletal champions fighting on foot, some with modern pike tactics and others with axes or swords, the usual mass of necrofeudal troops, lots of wizards, and, also, his cavalry.

Avistani cavalry tactics have not changed very much in the centuries since the Shining Crusade. You get on a big strong horse (or other big strong animal) with all the armor you can pile on the horse and onto you, and a pointy stick, and you ride very fast at the target and then the target either runs away or the pointy stick delivers of the mass of that metal directly to the enemy, usually fatally. The central elements of cavalry tactics - when to ride stirrup to stirrup for mass of charges, when to scatter to avoid fireballs, when to break and wheel - remain as they were a hundred years ago, or four hundred, or a thousand. And "Being undead" does not particularly change it, except insofar as it is much harder to find or make mounts for dead warriors.

A result of this is that his two thousand heavy cavalry, mounted on vampire-called wolves or animated barding over skeletal frames or jet-black horses with fiery eyes or stitched-together abominations of a dozen corpses, are actually basically the same as the Odranto cavalry except that the riders were in life and are in death the best Ksiaze could find.

(He does not use light cavalry for scouting. He uses familiars, invisible air elementals, incorporeal spirits, animated crows, and all the rest of the things you can use for scouting when you are not constrained by ordinary war.)

They have more clerics, more wizards, and a natural barrier. They've planted sharpened stakes as a shield against cavalry on their infantry's flanks, placed abattises to block the ford, raised pavises for the militiamen and some of their other soldiers to shoot from behind, and as many of the militia archers as will fit are in an old tower built to defend the ford, because they can't flee if they're already inside the most defensible position they can find.

At its core, though, this will be a battle won by wizardry and clerickry. Even the undead will have trouble crossing where there is no fords; without the weight of the flesh to anchor them, they're at more risk of being swept away than any ordinary soldier; with the advantage of the defensive and the advantage of the river, there will be more than enough time for a superiority in wizardry to play out to a decisive finish. The priests of the Living God have huge numbers of Fireball and Haste spells prepared, with wands and staves crafted by the Living God himself ready to supplement their resources. Their numbers may be few, but Ksiaze cannot bring his whole forces to bear across the narrowness of the Fords of Berus.

Oh, Ksiaze isn't leading this battle, Kasimir is. Ksiaze is just here to provide moral support and empty a few of his spell slots he doesn't need.

He does, after all, have two to one numbers, with troops who are functionally immune to arrows. He'll play their game. It's still day when he arrives, so he'll break open some bags of holding full of arrows, advance his archers in loose skirmish formation to the edge of archery range, and open a long-range archery duel while his cavalry "look for undefended crossings."

There don't seem to be that many undefended crossings, for some odd reason!

New Razmiran will return fire with crossbows and longbows and a few Acid Arrows, and its pikemen will raise their insufficient bucklers as a screen against the deadly rain, and suffer.

(And halfling sling staffs, where they have local halfling slingers.)

Yeah, Kasimir is shooting the halflings first.

New Razmiran has less archery firepower than the skeletal champions do! Funny thing about two-to-one odds, that way. The skeletal champions can retreat when they take too much of a beating, and can take quite a lot of fire before they go; most of them have trained for decades or centuries at working together, and know how to concentrate and direct fire. And, of course, they have natural armor, inhuman agility, implausible toughness, and having most arrows just go between the bones without hitting them. There's a lot of advantages to being undead.

(Also, once it's been long enough that Ksiaze is pretty sure any Resist Fire spells will have passed, he's going to start fireballing their most concentrated lines. If they cast more Resist Fire spells, he will, of course, wait an hour for them to go away and then go back to it.)

As long as they're sticking to long range, the skeletal champions cannot actually do much except deplete their ammunition reserves, actually. You need to fire a lot of arrows to kill a veteran soldier, and they do have clerics of Pharasma to provide healing. Neither side is going to be able to do much, with this archery duel.

Yep! Both sides are going to exhaust themselves and their ammunition supplies and (with a few feigned assaults as his most heavily armored- and shielded- infantry regiments fake a crossing attempt) hopefully some of their spells.

... Of course, his troops don't get exhausted. They're dead, you know. They can stand in all weather, hot sun and cold rain while arrows rain down, for days, not like the living troops. Especially not the militia troops.

You can't keep shooting forever. Cross and you die.

Oh, the point isn't to shoot forever, or fast. The point is to stop the troops of Razmiran from resting during the day.

Once it starts getting dark, the undead will start casting Light on the arrows they fire, just to mess with the Razmiran troops' night vision. 

And the Razmirani troops will cast Light on the sharpened stakes in the abbatis (well ahead of their troops, to illuminate undead crossing) and the front-center of the pavises, shining in the faces of attackers but not defenders. Same to you, jerks.

(One of the little-known facts about incorporeal undead is that they cannot go completely inside objects, though they can pass nearly their whole body through it. At some point, some of them must be outside the cover of a solid object, and in empty air.)

(One of the well-known facts is that almost all of them are vulnerable to sunlight.)

All right, do the undead intend to do anything? The Razmirani have much better supply lines.

Ksiaze will create an illusion of himself on his gryphon, and then, in the darkness lit only by cantrips, order his wizards to cast their prepared spells, and for his most-armored and best-shielded troops to be drawn up for a direct assault across the fords, ready to be propelled at Haste speed.

Any success finding undefended crossings, by the way?

Nope! The Razmiran troops have securing the fords under control.

Cast the buffs and go, shieldbearers first with shields raised high to shelter their heads from the arrows and fireballs, mass Resist Fire up, a mass of champions packed together tightly so the wind won't sweep them away, the illusory gryphon-lich swooping overhead to attract fire. (Along with various other illusions, including an undead dragon just on general principles.)

They're never going to have a better target for arrows or for area-of-effect spells. Open fire.