Earlier this week I finally caught up on where we are in the campaign (the one where I'm a player, not the one I'm running for my children). Essentially, we've brought powerful magical weapons back to the fort and they made a decisive difference when the massive army on our doorstep decided to attack the fort. (Between what we've learned here and what our earlier set of characters discovered, someone powerful is playing a very deep game; there's some legitimate reason to believe that this entire invasion -- composed of barbarian tribes, bugbear tribes, several mercenary troupes from the far side of the desert, at least a handful of girallons, and we're-not-sure-what-all-else -- is actually some kind of feint.) The would-be invaders, after being repulsed once, stayed put for three days; then the lord's Solarii (basically, a knightly-ish order for very-high-level characters of any class) scried that one of the larger mercenary companies was moving, and was sending expeditions to attack the gatehouses along the wall.
The available Solarii went with sections of the troops from the fort, one to each gatehouse -- except for the troops of Captain Sacha, under whose command our characters originally soldiered. Captain Sacha was being sent to the southernmost active gatehouse, and there simply weren't any more Solarii available to accompany him. So he appealed to our characters for assistance and naturally we went with him. (We're still technically soldiers of the fort, and drawing pay, but we'd been out on special assignments for several months, and our current place in the chain of command was... uncertain. So having Sacha request our services as a special unit actually kind of clarified our role within the fort and the Sol Povan military.)
At which point we got to try out our DM's latest attempt to combine mass combat/wargaming rules with regular Dungeons and Dragons. (We're using 3.5 edition, if you're curious and didn't know already.) It... actually worked pretty well, at least to my eye. He has a spreadsheet set up in Excel where he assigns a challenge rating to each unit (which are generally either 16 soldiers or 1 special/higher-level character) and when they come into conflict each side rolls a D20 and compares the results. Depending on who came out ahead, some bonuses for environmental factors (like, say, firing from a fortified gatehouse which offers height and cover) and whether the combat is reciprocal, one or both sides loses some percentage of their troops/hit points. (By "reciprocal" I mean that if you have archers firing down on advancing infantry from the top of a wall, the infantry don't damage the archers regardless of how they roll - but if they roll well, they lose a lot fewer people. But if if you have two infantry units in melee, it's reciprocal and both sides take casualties.) It's basically a way to have the characters participate in a large-scale battle, but one where -- because of their comparatively high character levels and the way that affects personal power in D'n'D -- they can still affect the overall battle with their individual actions.
And while it's not perfect and it still has some wrinkles to iron out, it basically works.
(The other approach, in general, is to reduce the overall battle to a collection of smaller encounters with the larger battle serving essentially as the background/setting for those scenes. From a writing perspective, you can also sort of write about the opposing armies and/or individual units as opposing characters, but that doesn't translate well into the typical D'n'D experience. It could probably done if somebody cared to put in the time, or if you did something akin to switching out to classical wargaming for a session, but I don't know of any native systems for it. This approach, while not perfect, does a surprisingly good job of bridging the gap between those two.)
So: fighting from behind/atop a fortified wall with five units of line soldiers (four of which could also serve as archers) and their five lieutenants and the captain also acting as one-individual units, our... 80 line soldiers plus 4 ballista plus 9 exceptional individuals annihilated some 400 would-be invaders, plus their general (who should have been a more serious threat, but he had planned to approach in an armored chariot and we killed the dire boar pulling the cart with a ballista shot in the first round), plus two girallon using tower shields who were effectively siege towers in their own right. We lost, I think, roughly half of our troops, including one of the lieutenants. It's been a long time since I considered majoring in Medieval Studies, and the power imbalances in D'n'D skew things more than a little, but that seems about right to me for infantry assaulting a fortified position with one battering ram and two (monsters who might as well have been) siege towers.
We concentrated on the foot on our first round, to slow their advance; when their archers responded, we realized we needed to take them out in order to survive long-term, so on the second round we switched (very effectively) to attacking troupes of archers. The general lost his armored dire boar in the first round, and wound up huddled in his armored battle chariot for most of the combat, until we'd killed enough of his troops to allow the ballistae to start making it a target. Clearing out the archers let the infantry advance relatively unmolested (after the first round) but once the archers were gone there nobody left in a position to damage us and we were able to whittle down the advancing infantry until they reached the walls (and, actually, even after). The girallon were tougher, and were the first up the wall -- like I said, they were essentially siege towers, except they were actually monsters -- but we were able to wear them down, mostly from a distance. The fighter managed to trip the first one when it made it to the top of the wall, and while he took a lot of damage from it that probably kept everything from going horribly wrong.
By the time the general realized we were smashing his shelter and charged the wall, it was too late. The archers were busy elsewhere; the people shooting at him were the ones aiming the ballistae. He... didn't last long.
And my elvish rogue/ranger (who is quite effective as an archer) got to wipe out a couple of companies by himself, from atop a tower behind the walls (Legolas-style, I rather imagine) so I was quite pleased with the session, as well as the outcome.
Shortly after the battle ended, one of the Solarii arrived from the next post up. He'd come to offer aid that we clearly didn't need, and bring warning of an elite group that was apparently planning to tunnel beneath the wall at the southern end, where the final gatepost had long ago been sealed up. So we surrendered the soldiers of the line to his command, took the captain and his lieutenants with us, and headed south to deal with this new threat.
We slept first. Even with magical healing, you do that after a battle.