Fort Fellhammer isn't difficult to find, lying not too far from the main road to Dawnstar. Giren, having slipped out on his own during the scuffle of words, is the first to arrive. His quick survey of the structure shows it in an advanced state of decay, with bricks tumbling out of the walls like acorns from an autumn oak. At least one part of the wall has collapsed in on itself, but both the gap and the gate have makeshift palisades guarding the entrances. The disrepair of the building has suited Magnus Grim-Foot's men just fine - the structure is too broken to be worthy of attention from the Legion or the Stormcloaks, but provides more defense than the typical dug-out or co-opted cave.
Giren can see bustling activity within the walls - he counts at least 5 men patrolling the walls themselves, and maybe 10 or 20 more inside the courtyard, sitting around a large bonfire - it's hard to attain an accurate count, because the bandits enter and exit the garrison freely to escape the cold. As he finishes his scouting, the rest of the Nightgate patrons arrive, first in a relatively large group, and then any stragglers who either took longer to make up their minds, or simply preferred their own company for the march to war.