The attacking army is composed of about 50,000 troops. These are mostly infantry, although we have not been told their armament. There are also some archers, although no estimate of their number has been given. Likewise, there are an unknown number of Ganeshi Moonriders. However, the Moonriders dislike fighting during the day and, in any case, have been fighting all night (not just the opening battle with the Army of the Valley, but also ranging around Garnath ambushing patrols and generally causing trouble). Finally, there are is artillery which fires flaming projectiles a truly distressing distance. We have no data on what they are or what their exact range is, although, given they bombarded the Garnath Fort for several hours, it's probably same to assume that they have plenty of ammunition.
Amber's forces are divided into three groups under the command of Marshal Caitlin, Admiral Grace, and Prince Dexter.
Caitlin is defending the approach to Castle Amber with the bulk of Amber's army, roughly 15,500 soldiers. This force includes approximately 7,500 infantry, 5,000 of whom are fresh plus the survivors of the Army of the Valley (estimated at about 2,500 of the 5,000 infantry assigned to the Army), who will have been under bombardment for much of the night and have just fought a long engagement with the Moonriders and then marched across much of Garnath. The infantry are backed by 3,000 archers, all of whom are fresh and ready for battle. There are also about 2,500 cavalry, 1,000 of whom are fresh and the remainder of whom have just engaged the Moonriders (estimated that 1,500 of the 3,000 cavalry involved in that battle survived). The cavalry are backed by 1,500 horse archers, 1,000 of whom are fresh while the remainder (estimated at 500 of the 1,000 archers involved) are survivors of the Garnath battle. Finally, there are 1,000 field engineer/guerillas.
Grace holds Amber City with a garrison of 2,100 soldiers. These include 300 city watch and just under 1,800 marines.
Dexter is in Arden raising the Rangers. No one save Dexter himself knows how many Rangers there are, but Amber's official battle plans call for a force of 10,000. Given the odds she faces on Kolvir, Caitlin fervently hopes Dexter hasn't been exaggerating the size of his army.
The 200 Castle Guard remain on watch on the walls, alert for anything that might slip past Caitlin's force on Kolvir.
Amber can call upon some 23,000 troops from near Shadow, including a feudal army 13,000 strong (mostly pikemen, who would have been nice to have against the Moonriders) and some 10,000 horse nomads who would love to test their mettle against the Moonriders. They are probably too far away to matter, but Caitlin intends to send for them, just in case. Better to call them and have them arrive too late than to need them and not have them.
Dierdre and Martin may or may be available and may or may not have paranormal powers of use in the battle.
Amber faces three significant disadvantages. First, the main body of its forces are outnumbered by more than three to one. Second, it's army must defend two widely separated fixed points (the City and the Castle) without knowing the enemy's objective. Third, the enemy has highly destructive long-range artillery of unknown nature which makes withstanding a long siege (which might otherwise be the preferred tactic under these circumstances) impractical. The first two (assuming Jim's Benedict supposition is incorrect), we have some hope of counteracting through the superior skill of Amber's generalship. The third may be somewhat more problematic.
Caitlin will place her army along the flank of Mt. Kolvir on the approach to Castle Amber, such that the enemy army has to go through her to reach the Castle. If possible, she'll deploy behind (in order of desirability) a small stream, a gully, or a steep slope. Basically, she wants a terrain feature that will be difficult to cross, slowing and disorganizing their advance and hindering the mobility of their cavalry. If possible, she'd also like to run her line across a valley (perhaps the road cut?) in the mountain or similar terrain such that the natural lay of the land will make it difficult or impossible for the enemy to get around the flanks of her army. Regardless of other aspects of the terrain, the Amberite forces will have the advantage of elevation over the attacker, reducing the momentum of the attacker's forces, increasing the momentum of any counter-charge and making the attacking archers work against gravity while Amber's archers have gravity in their favor.
The infantry will line up behind whatever terrain feature we selected as our defense line to take advantage of the enemy's expected difficulties in crossing it. The archers (including the horse archers) will be deployed up the mountain, such that they will be able to rain arrows down on the enemy forces as they approach and then cross the terrain feature. The horse archers will line up behind the foot archers to facilitate shifting them quickly as the battle develops. The cavalry will be held in reserve, to be committed when and where it appears that one of the lines is faltering, either as support for an embattled position on our line or to exploit a weakness in the enemy line. Note that the cavalry does not have to be committed as a unit; brigade-sized units to deal with individual situations. However, a full division will be held back for "the big push" at a key moment. If there is any convenient high ground overlooking the attacker's expected line of march, the guerillas will be deployed there to harry the enemy army as it approaches. If not, they'll deploy in small groups all along the lower flank of Kolvir. Once the battle is joined, they are to move out behind the enemy army and cause what disruptions they can. The siege engines are to be particular targets.
And speaking of those siege engines, prior to the approach of the attacking army, will be kept dispersed across the potential battlefield to minimize the damage that the attacker can do with long-range bombardment. Only when the enemy begins to advance will the defenders form up and prepare to meet them. The cavalry will be kept dispersed until it is committed. In a similar vein, headquarters units won't be marching with their banners, so as not to present obvious targets for the artillery (I realize this can be bad for morale, but getting napalmed is worse). In fact, while waiting for the enemy army to come up, they can plant some banners with nobody around them to try to draw artillery fire away from actual soldiers.
I'm not sure exactly what the Amberite tradition for commanders is. Caitlin would prefer to command the reserve, directing the flow of the battle as it develops and using her own judgement to find the critical moment to commit the cavalry. However, as this is her first major engagement and the odds are badly against Amber, she is very aware of the importance of morale among her troops. If tradition (tradition, in this case, meaning what Martin did as Lord Defender and what Galen did not do) places the Defender at the head of the army, then that's where she'll be, leaving command of the reserve to another (Root would be ideal for this, if he's available; otherwise Julianna of the Shadow Guard).
Of course, all this assumes that the enemy is willing to fight the battle Caitlin wants to fight. Such may not be the case. Several other possibilities occur:
The enemy may turn and attack the City. In that case, Caitlin will wait for Dexter to arrive with the Rangers, link up the two forces, then attack. It will be up to Grace to hold the walls until that can take place. In this situation, Amber will not have terrain on its side. However, the city is large and the attacker will have to disperse his forces (which will be less mobile infantry vs. Amber's largely cavalry force) around the city walls. This, combined with the link up of the two forces should provide Amber with local parity in numbers, if not actual superiority, as they try to sweep around the walls. In addition, the fact that the bulk of Amber's army will not be defending fixed positions, will render the siege engines much less effective (although they're not likely to help property values in Amber City much).
Alternatively, the enemy simply take up a position on the plains outside the City (where the road from Arden meets the road from the Castle would be ideal for this), use their artillery to pound the Castle and/or the City into rubble and dare the Amberite forces to come do something about it. Ideally, if this happens, Caitlin would like to sit and take it for a couple days while the armies from Shadow are brought up. When that happens, the odds will be even and Amber will have the advantage even in a battle on the open plains. However, the City or Castle may not be able to survive the time required for that to happen. In that case, Amber will be forced to act as if the City were being attacked directly, but the enemy will probably be much better prepared and able to amass most or all of its forces to meet Amber's army. I don't think there is a good plan for that situation.
Root
Currently not in Amber. If he can be returned, Caitlin will ask him to join her command of the main army. If she ends up leading from the front, she'll put Root in command of the reserve. Otherwise, he can command one flank of the army. Given the disposition of enemy forces, Caitlin doesn't think it necessary that he stay in the Castle but, if he insists, she won't press him.
Dierdre
Location unknown. If she can be found, Caitlin hopes that she'll have some sort of supernatural powers she can bring to bare to help in Amber's defense. Otherwise, she should stay with Martin and prepare to get him out of Amber if things go badly.
Martin
If Martin has some heretofore undisclosed powers (Jewel of Judgement?) that can help, he's welcome to use them from as safe a distance as possible. If not, he should stay in the Castle, safe and out of the way.
Galen
From his (brief) tenure as Lord Defender, Galen ought to know where the Shadow armies are. That would make Galen the idea candidate to go get them. The only difficulty will be the 50,000 enemy soldiers between Castle Amber and the shortest route to the Shadow armies. Perhaps Dierdre could help with this. Perhaps Martin secretly has a Trump that would be useful. Perhaps Galen could Walk the Pattern.
Gustav
There are two Shadow armies. One can hope Gustav also knows where they are (or could be sent to one even without that knowledge). If so, he can bring one back while Galen goes and gets the other. If not, Caitlin has a whole bunch of Trump sketches she'd like made.
Alexandria
She should be aware that the Castle is likely to come under rather extensive attack from flaming projectiles and should prepare the Castle accordingly (move anything flammable indoors, wet down anything that can't be moved, prepare fire crews, etc.).
Carton, Mirabelle, Theo
We don't know where they are and we don't much care.
Elanora
Assuming Martin isn't running around on the battlefield, she should stay with him so that they'll be together if the Castle has to be evacuated.
Message sent by courier to Amber City for Admiral Grace
Grace,
The defense of the City is in your hands. I have stationed you there so that you will be close at hand to the fleet if a naval attack comes. If so, take what you need and meet it, leaving the defense of the City to Capt. Hyde. If no naval force materializes and the enemy bypasses you, you may sally forth to harry the enemy, as long as doing so does not put the city at undue risk. In particular, anything you can do to their siege engines would be of help. Speaking of siege engines, prepare the city for heavy bombardment by flaming projectiles. Form fire brigades, wet down the roofs, etc. If you are unable to hold the city, make sure they don't capture your Trumps.
--Caitlin
Message sent by courier to Arden for Prince Dexter
Dex,
I intend to meet them on the slopes of Kolvir. Let them engage me, then move to strike in their flank or rear. You could either target their fighting forces immediately or go for that artillery first. I lean towards the artillery, but I trust your judgement. Just like the old days in Arden. If plans change, I'll Trump you.
--Cait