Edgar knows the keep in question from stories, even before the commander mentions it.
"Quandry Keep was built to befuddle invaders, the same way all of them were before things got... 'civilized'." The air quotes are audible.
"There is a seal that can point the way, sort of an arcane compass, but it went missing the same time the keep did. If they're making any effort to take the keep though, they'll need it, because the only other way to not get lost in there is to not go in in the first place. You might need to make a detour to obtain that seal for yourself. We have a group of soldiers guarding a void bomb down by the front lines. They're currently under orders to use it if the lines break, but you'll have orders for them to hand it over to you. It's not large; you should be able to carry it with you easily and plant it in the keep to bring it down."
There's another way too, Furmyr knows. Something the dwarf is keeping from you all, probably because you're NOT dwarves, and he's paranoid about non-dwarves knowing. In every keep, there's glyphs on the walls at certain intersections that can be read to figure out where exactly you are, sort of a grid system, but if the keep was deserted officially, some of those might be missing, having been scraped off by the retreating soldiers. Still, it could be a better option than trying to figure out where the derro are keeping the seal.
OOC:
I'm not putting that in a note because there's no real point in keeping secrets about what everyone knows UNLESS you would rather keep it secret. If that's the case, please let me know now, and I'll start doing it. I've found it slows things down more than accomplishing anything else, but I will certainly bow to the group's wishes.