Durin believes the dwarves are beginning to turn the tide of battle in their favour, and he wishes to lead a charge up the steps to face the Lord of Gundabad and claim the fortress.
'One obstacle remains, my friend. We must do something about Gorgar the Ruthless, the Orc who rules Gundabad. His Orcs will defend the Mountain-home from us for as long as he lives, but if I can put an end to him they will scatter. They will learn first-hand that only a dwarf can lay claim to the title Lord of Gundabad!
'Blow the signal horns to rally dwarves to my banner: outside Zarkul-sulûn, in the field of battle, and at our forward encampment nearest to the Mountain-home! I will lead my warriors up the steps on this last offensive, and on the very threshold of the fortress I will challenge Gorgar for the rule of Gundabad!'
Durin asked you to use the signal horn outside Zarkul-sulûn to rally the dwarves for his assault upon the threshold of Gundabad.
Durin asked you to use the signal horn at one of the trenches to the north-west of Zarkul-sulûn to rally the dwarves for his assault upon the threshold of Gundabad.
Durin asked you to use the signal horn at the hidden cave of Drenghól, nearest encampment to the Mountain-home, to rally the dwarves for his assault upon the threshold of Gundabad.
Bróin is in the hidden cave of Drenghól, nearest dwarf-encampment to Gundabad.