In my campaign, I have determined the kingdoms of Wessex and Essex will be jointly invading Salisbury in 503. Anyone have an idea about the KV (Knight Value) of an invading army for this period? The KV of Salisbury will be about 80, I think (70 KV for the Knights of the county (not counting footmen) and about 10 KV for the player knights.

I don't think the current publications use the KV anymore. However, I do! :P

See this thread for similar question and answers there-in:
http://nocturnal-media.com/forum/index.php?topic=2476.0

I estimated Wessex' strength at 2000 warriors based on the 508 Battle of Netley March, when they are defending their homes. I'd imagine that an invasion army would be smaller, but depends a lot how keen they are on crushing Salisbury.

I think Essex is stronger than Wessex during this period: they seem to have more land and have been established for longer. So I'd say somewhere in the 3000 - 4000 range.

Sussex has AT LEAST 5000 (508 BoNM again), possibly even 6000 (to have a bit of a home garrison, too). But they are stated to be the strongest Saxon kingdom, too.

Now, how to convert warriors to KV? It really depends what you think the basic Saxon is worth. The Saxon Raider template in the KAP 5.1 with 5d6+1d6 Axe-14 is certainly more worthy than the foot soldier at skill 10, damage 3d6 (too weedy for me, my professional foot soldiers tend to be skill 12, damage 4d6, to make them actually better than a mere bandit). However, I tend to use Saxon Ceorls (the basic freeman farmer-warrior) with 5d6 damage and Spear 12 as the mainstay of the Saxon battleline, with those Saxon Raiders as the more 'elite', full-time warriors. So my calculations go like this:

Common peasant militia: no armor or training, just improvised weapons or at best, a spear = 1/32 KV
Bandits: some skill, poor or no armor, spear or some such = 1/16 KV
Foot soldiers, archers: trained, armor (and shield for spearmen), proper arms = 1/8 KV
Armored foot soldiers, crossbowmen: skilled, heavier armor and/or better weapons = 1/4 KV
Mounted Sergeants: skilled, heavy armor, on horseback with lances = 1/2 KV
Knights: skilled, heavy armor, on chargers with lances = 1 KV (particularly skilled or well-armored knights might be more)

Saxon ceorls (spearmen militia) would be equivalent to foot soldiers, so 1/8 KV.
Saxon raiders (axe-wielding full-time warriors) would be equivalent to armored foot soldiers, so 1/4 KV.
Saxon huscarls (bodyguards) would be equivalent to mounted sergeants, so 1/2 KV.
Saxon berserkers would be equivalent to knights, so 1 KV each. Particularly fierce berserkers might be 2 or even 3 KV.

To take this back to the original question, I'd say that the invasion armies might be, at worst case:
Wessex: 500 raiders and 1000 ceorls (leaving 500 ceorls as garrison; the army is close to home) = 250 KV
Essex: 1000 raiders and 1000 ceorls (leaving 2000 ceorls home, due to the distance) = 375 KV
(Ignoring thegns (chieftains) and berserkers, who would only be a small proportion of these... you could adjust the KV by +10% - +20% if you really wished to and it would be close enough.)

As you can see, Salisbury is boned if those two armies come at them. However, will those armies be able to send all those warriors at Salisbury? Do they have other enemies, Cymri or Saxon?

Also, Salisbury would have foot soldiers, too, 3 per knight. That is additional 0.375 KV per knight so 30 KV to add to the 80 you mentioned. That makes it 110 KV. Raising the peasant levies to fight against the invaders, at roughly ~50 / manor (based on the character sheet's Army section), would actually boost the numbers considerably, to about +1.5 KV / knight = +120 KV. So mustering all the manpower of Salisbury would result in a defensive force of 230 KV, capable perhaps to fighting each of those armies individually as long as they wouldn't have time to unite. If they come together, then bad things happen. Also, those peasants wouldn't be able to be on the field for a long time, and not march quickly, so gathering all of them would be difficult, too. As might be convincing peasants from Western Salisbury to march all the way to the east to fight Saxons who 'wouldn't raid this far anyway'... And actually keeping them on the battlefield if going gets tough: the morale would be rather shaky.

The other option, and probably the safer one, would be to bring all valuables, noble families and footmen to Sarum (and other castles) and prepare for a siege, while the peasants scatter into the forests and most of the knights (60 or so?) harass the invading armies and pick off foragers and raiding parties one by one. Sure, the damage to the manors which happen to lie on the main invasion path will be horrific (might as well destroy the crops yourself, to deny them to the Saxons, scorched earth strategy), but most likely, the Saxons will be unable to stay long enough to press home the siege: Essex' Army especially would be starting to feel the hunger soon enough, if they can't forage efficiently in Salisbury.

But yes, if Salisbury has managed to piss off two Saxon kings enough for them to cooperate in a joint invasion on Salisbury, then Salisbury is in deep doo-doo can can only blame itself (or the PKs, as the case may be). Do they have any allies to call upon?

Wonderful, wonderful answer Morien. Exactly what I was looking for. I'll reply with more detail tomorrow, when I have some time.

Alright, I run a G+ campaign for about five players. We play every two weeks, and have about 35 sessions under our belt. The current year is 503.

The players (being players) have refused tribute every year it was demanded. That's eight years of refusals for Wessex, and many other refusals for the other Saxon kingdoms.

My variant grudge chart mechanics dictate that this year there will be invasion from Wessex, and pillaging (or large scale raiding) from Sussex & Essex.

On this basis, I assume there will be in invasion led by the Atheling of Wessex, Cynric with assistance from Sussex & Essex.

The invasion route will be along the royal road from Winchester to Du Plain Castle, and then to Sarum. The forces from Sussex and Essex will travel though part of Silchester, in the territory controlled by the Steward of Levcomagus (since Sussex and Silchester are allies at this time, I have assumed that this would allow for the normally unthinkable event of of Saxon troops crossing a British kingdom to attack another British Kingdom) and attack along the road through Grately. I have also told the PCs that there are strong raiding parties from the Camelot Forest and the Harewood, which means bands of pillagers who have separated from the main armies.

So not a full court press, but a large portion of the available troops from Wessex (50% for an invasion led by the heir?) and about 20% of the available troops from Sussex and Essex.

Applying these assumptions to your numbers, I would assume the following: 125 KV force led by Cynric, a 40 KV force led by Kelan of Sussex, a 35 KV force led by Aescwine of Essex, and six marauding bands (3 for each forest) of 12 KV each, for a total of 72 KV. That's a total of 272 KV. The plan will be to link up at Sarum, after taking Du Plain and plundering the western side of the county. If no army issues from Sarum, they will plunder the north and the east.

I use a very loose version of the Book of Battle rules, and I will use the army commander rolls to determine how many KV are lost each round of battle (success on AC roll, inflict a 1d6 loss on opponent, double for critical and 1d6 loss on a fumble). I'll use the simple siege rules from the GPC.

So, how does that look to you? Any questions or comments?

Hmm. After 8 years of refusing to pay tribute, I would hammer Salisbury harder, especially as Wessex. They are taking a bit of a risk of marching in with a force that is roughly comparable to Salisbury's, counting just the knights and footsoldiers. That can easily be inviting a bloody nose and a very bad precedent. Even after the armies join up, there is no way that they can crack Sarum. They have just 2:1 odds vs. Salisbury on the field, and behind the walls, Salisbury's armies will be safe. Even assuming they stay there.

A lot depends on the surprise. If the Saxon armies march quickly on Sarum, they might be able to link up and thus present a strong front vs. Salisbury. However, reading your 'battle plan', they intend to take Caer Du Plain (might be costly, as it is a castle) and plunder the western half of Salisbury, before linking up. Pillaging tends to take time (weeks, if I recall ESTATE right), so this would give the Salisbury's army a chance to defeat them in detail.

Especially the Essex+Sussex contingent (assuming they march together) are vulnerable. If Salisbury's army catches them, especially while they are spread out to plunder, they can be mopped up with local superiority of numbers. Heck, I would be tempted to just take maybe 60 knights or so and try to catch those two princes separately and capture them. Might be easier said than done, but probably the best chance of actually managing to get a peace of sorts. Of course, some of the knights and footsoldiers might be unavailable, having been surprised in their manors or at Caer Du Plain. If the Salisbury knights have advance warning of the Saxons coming, they are well poised to hammer them hard once they get the chance. With Essex and Sussex out of the way, those other raiding parties can be ridden down if they come out in the open, and then the focus can shift to the Wessex group.

So yeah, there is definitely a chance for the PKs to turn this thing around. I wouldn't have been quite as merciful, like I said. :P