Page 62 in Hellknights article in What Lies in Dust says the Order of the Nail moved from Citadel Altaerein east of Logas.
Page 67 in Path of the Hellknights artice in The Infernal Syndrome says the Order of the Nail moved from Citadel Draythe near Remesiana.
|“||Okay. So. The former headquarters of the Order of the Nail was Citadel Altaerein in Isger. While "east of Logas" is correct, the map of Isger in the Inner Sea World Guide shows that heading east from Elidir is a slightly more expedient way of getting to the Citadel.
Citadel Altaerein was constructed in Cheliax's Isgeri holdings because of the region's issues with savage races - goblins and hobgoblins in particular. As the Order of the Nail directly opposes savage cultures, a fortress close to the battle lines between civilization and the wilds seemed optimal.
That said, Isger is not the only realm in Golarion with a problem with savage cultures. To facilitate their movement beyond Cheliax's borders, the Order of the Nail also maintained the much smaller, but still heavily fortified wharves known as Citadel Draythe near Remesiana. While not the command center of the Order's lictor, it did serve as the disembarking point for questing Order of the Nail Hellknights heading forth to frontiers like the Mwangi Expanse, the Land of the Linnorm Kings, Qadira, and - fatefully - Varisia.
When the Order of the Nail was convinced to relocate its headquarters to the Varisian frontier, most travel passed down the Conerica River to the River Iseld and ultimately to the Order's holdings at Remesiana. From there the Hellknights debarked to Varisia, where the majority of their Order remains to this day. Both citadels continue to operate as Hellknight holdings, though with only small garrisons.