The Slayers (BtVS) at Helm's Deep.

[The Romulan Republic]

So, for this scenario, we're going to give an unexpected addition to the defenders of Helm's Deep. Film version for Helm's Deep, partly because its easier to get a sense of the strengths and weaknesses of the defences from visual images than text.

Version one:

The Slayer Army from the finale of Buffy the Vampire Slayer is transplanted to Helm's Deep, augmenting the existing defenders. Can they hold the wall and minimize casualties?

Version two:

Same as above, except the Slayers replace the defenders. They must hold the keep and prevent the slaughter of the civilians within the caves behind it, until Gandalf and Eomer arrive to break the siege, then aid in breaking the siege.

For the record, the forces present are:

Helm's Deep Defenders: 300 Rohirim fighters on foot, many conscripts, mostly old men and boys. A mix of bowmen, spearmen, and melee infantry. Also present are King Theoden and a handful of his horsemen, Eowyn the shield maiden of Rohan, and three members of the Fellowship of the Ring: Aragorn (charismatic leader and expert swordsman), Legolas (superhumanly accurate and agile elvish bowman), and Gimli (dwarvish melee fighter). They have also been reinforced by an unknown number of armoured elves armed with bows and swords (probably hundreds in number).

Helm's Deep Reinforcements: 2,000 cavalry under Eomer and the wizard Gandalf (notable abilities include speaking to animals, vast knowledge of lore, telekinesis, psychic combat, and magical evil-repelling light beams, as well as melee combat). They will arrive at dawn (its not clear when the battle starts in the film, but some time, probably hours, after night fall-we'll say around midnight, since I think that's what it is in the books).

Besieging Army: At least 10,000 Uruk-Hai, mostly pikemen and swordsmen but with substantial numbers of crossbowmen. They have brought explosives to bring down the wall, as well as a battering ram (protected by a shield phalanx), many ladders, and a few ballistas.

Slayer Army:

Two Slayers (Buffy and Faith), possessing prophetic dreams, low-end superhuman senses and reflexes, and low-end superhuman strength, speed, healing, and durability (against blunt trauma and electrocution, at least- they don't seem to hold up much better than ordinary humans against piercing or cutting weapons). Armed with melee weaponry. Buffy has the Scythe.

Approximately twenty Potentials (girls who could become Slayers), who possess some training in melee combat and are armed with a variety of melee weapons.

One powerful witch (Willow). Notable abilities include, but are not limited to: summonings, tracking spells, rapid healing and healing spells, flight, conjuring small objects, telekinesis, mental attacks, casting lightning bolts, teleporting an opponent (though she had help with that one and it physically injured her), and raising a shield around a building strong enough to keep out a small army (say, a few dozen troops) for about a day. Holy men can apparently bring down said shield faster. She is often reluctant to use her full powers, and risks going evil when she does, but she is currently preparing a ritual to activate all the Potentials using the Scythe.

One vampire with a soul (Spike). Skilled melee fighter with low-end superhuman senses, speed, strength, healing, and durability (vampiric vulnerabilities to fire, wooden stakes through the heart, beheading, garlic, Sunlight, and holy water/holy objects aside, and other specific magical threats he is unlikely to encounter here aside). Has an amulet which can wipe out an army of super vampires and a Hell Mouth, destroying him in the process. However, assume that it will not activate in this battle.

Six mundane human warriors of varying degrees of skill and experience (Giles, Wood, Anya, Xander, Andrew, and Dawn). Armed with melee weaponry.

Edit: I specified only holding the keep for the second scenario because I presume that they simply don't have the numbers to man the keep and the entire wall.

[FaxModem1]

Can any of the slayers or other transplanted defenders, being from the 20th century, pick up on the gunpowder bomb before an orc with a flare runs up to it blow a hole in the wall? That might cause a significant difference in how things go.

[The Romulan Republic]

Can any of the slayers or other transplanted defenders, being from the 20th century, pick up on the gunpowder bomb before an orc with a flare runs up to it blow a hole in the wall? That might cause a significant difference in how things go.

Better yet:

It just occurred to me that The Two Towers hit theatres in late 2002, while the finale of Buffy was in spring 2003. And we know from dialogue (Andrew) that some version of LotR exists in the Buffyverse. Which means that their is a very real possibility that at least some of the Slayer army has seen the film a few months before.

So they will know where they are (and probably not be as freaked out by it as most would be, given that some of their number of experience with inter-dimensional travel), and what to expect.

So yeah, orders are likely issued to shoot any Uruk-Hai running at the wall with a sparkly torch.

Edit: Also, Faith and a bunch of the potentials got bombed an episode or two earlier, so they'll likely be extra twitchy about the possibility of explosives in unexpected places.

[The Romulan Republic]

Really, for scenario one, I think the boost in manpower is probably less significant than any knowledge to expect explosives. After all, a Slayer or vampire, one on one, isn't really more impressive than some of the elves in the LotR and Hobbit films.

The character interactions will be interesting though. Especially with female warriors being a rarity in Middle Earth. I wonder what Eowyn's response will be to the Slayers.

For scenario two, and to a lesser extent for scenario one, the big question is probably how good Willow's shield magic is. If she can put a barrier spell up around the whole citadel, and keep it up all night (admittedly a stretch, but maybe possible for season seven Willow), then they can potentially win with zero losses.

Failing that, if she can put a barrier spell across the entrance to the innermost tower, they can stymie the Uruk-Hai advance their. However, this will allow the Uruk-Hai to take the outer walls of the citidel, meaning that the defenders will have to sally at dawn, or the reinforcements will have to storm their own fortifications to free them from the Uruk-Hai. Still, I think a surprise charge from a couple dozen Slayers can match Theoden's sortie, even though the Slayers, like the other defenders, are an infantry, not a cavalry force. Of course, the Uruk-Hai are an infantry force as well.

Of course, Spike could not take part in any sortie at daybreak.

Another point that I should clarify- while I specified that the Slayer army is a melee force, this is based off the weapons they took with them into the battle in the Hell Mouth in the finale. Some of them are certainly proficient in the use of crossbows, and Faith is a capable archer as of season 3 of Buffy. Their is no reason, if the defenders have some spare bows, or they can capture some off of the fallen, that some of the Slayers cannot be armed as archers.

[FaxModem1]

Willow created a shield that guarded a gas station from the Knights hunting Dawn for a day or so in season 5, but this was also the season in which Spike could just wear a thick jacket over his head to prevent burning the sun, so take that as you will.

[The Romulan Republic]

She created a similar shield in season seven (albeit on a smaller scale and for a shorter amount of time) against the Turok Han in mid-season seven. As I recall, her letting the shield drop was also a ruse.

Though her shields seem to be vulnerable to sufficient kinetic force. I'm not certain, but didn't Glory basically punch her way through it (though it came back up afterward)? If so a thousand Uruk-Hai changing into it might well crack it.

[Solauren]

Willow barrier, Season 5: Glory was a literal Hell-God when she punched her way through the barrier put up. She was also on the inside, where the barrier was not meant to stop things.

Willow Barrier, Season 7: Hard to say what her limits were at that point. Less then a year ago, Willow nearly destroyed the whole fucking world powering up an artifact.

Willow is a very powerful telekinetic and using magic, she has hurt a Hell God. Remember her pissed off and going to Glory "You owe me pain?", and then giving that pain to Glory? Willow hurled dagger into Glory, and hurt a god in the process, with the proficiency of Darth Vader teaching his boy a few things on Bespin.

If Willow saw the Uruk-hai bomb, she could toss it, and the poor bastard carrying it, back over the head of the incoming army, and blow it back there.

Hell, it wouldn't surprise me if she started tossing the Uruk-hai back, and their weapons, back into the rest of the army.

At LOTR's tech and magic level, Willow, as a spellcaster, it's a weapon that wins battle, she's a weapon that could change the course of a war.

[The Romulan Republic]

Its hard to gauge Willow's power-level in season seven, because she's so often holding back out of fear of going evil.

I think we can probably say that she can duplicate any season five/early season six and season seven feats if sufficiently motivated (possibly at risk of going evil), but not necessarily the nastier/higher end stuff she did as Dark Willow.