"How is the mine situated? Is it built into the side of a hill, or is it mostly just a hole in the ground?"
"Built into the side of a trio of hills actually, three entrances come up onto the surface with a half circle ring of earthworks facing out from those entrances with gaps between them for two men to walk through side by side. The earthworks themselves are most likely around chest height by now with ditches in front of them unless the Falmir started slacking off and left their defenses as is, at least that is how I'd arrange the defenses if I had to hold such a position."
With that the pair of you arrive into the Disciples camp, as the men turn to look at you expectantly you make your way towards the center of the encampment causing the men to gather around to hear what you would have to say.
"The Falmir have betrayed peace talks, the have sent back the head of a child of the Banigar back in a box, and now Lord Vandir has chosen to go to war against these heathen Falmir, these worshipers of the old False Gods. Tomorrow the main army will lay to siege Castle Greymont, but the battle will begin tonight with us for the Lord Vandir has seen fit to grace us with the task of taking and holding the only route into and out of that Castle once the siege starts. To the northeast lies a mine which the Falmirs' soldiers use even now to smuggle more supplies and men into the Castle, ready yourselves for a march men for we take that mine tonight in the name of the King, for the Banigar!"
As you finish the last part your men echo the saying,
"In the name of the King, for the Banigar!"
The men continue to chant that line for a minute or two before dissolving to finish preparations, and ready for the march. Now to finalize how to take that damned mine ...
1) Seize the Mine from Falmir Forces
2) Hold the Mine to cut Castle Greymont off from additional supplies and reinforcements
12 Knights [Plate Armor, Lance, Arming Sword, Kite Shield, Destrier]
10 Footmen [Leather Armor, Short Sword, Round Shield]
4 Squires [Chainmail, Arming Sword, Round Shield]
5 Crossbowmen [Leather Armor, Crossbow, Shortsword]
2 Long-bowmen [Longbows, Hand Axes]