Many years passed, until one day a great barbarian invasion came from the lands to the north, threatening to engulf the entire land with the savage excesses of the unchecked alien horde.
Module B1 "In Search of the Unknown"
While the default setting for module B1 "In Search of the Unknown" is generic, advice is given for situating the Caverns of Quasqueton within the World of Greyhawk:
In the mythical WORLD OF GREYHAWK (available from TSR) the stronghold can be considered within any one of the following lands - the Barony of Ratik, the Duchy of Tenh, or the Theocracy of the Pale.
Potential locations for module B1 in the World of Greyhawk (1980). The Thillonrian Peninsula to the north is home to a number of barbarian kingdoms, who "consider the Great Kingdom and the Sea Barons as their most natural source of easy loot and profit."
The original World of Greyhawk folio describes the Flanaess, the northeastern part of the continent of Oerik, and includes a beautiful map by Darlene, based upon the map of the Great Kingdom of the Castle and Crusade Society:
The map of the Great Kingdom, from Domesday Book #9. Note the northern peninsula, home to the "Northern Barbarians".
Dave Arneson situated the Barony of Blackmoor at the southwestern end of the Great Bay on the map of the Great Kingdom, and mentions in "The First Fantasy Campaign" that Blackmoor Castle was originally constructed as a defense against barbarian incursions:
Blackmoor Castle was built in the third year of the reign of Robert I, King of all Geneva, as a defense against the Barbarian hordes that periodically sweep through this area over a period of six years. On the hill that dominated the small village of Blackmoor, there are the ruins of several previous structures that were destroyed by the Barbarians.
JG 37: The First Fantasy Campaign
Gary Gygax also mentions Blackmoor in his story "The Gnome Cache" serialized in The Dragon (#1-3, 5-7). In the final installment to be published, the "border keep of Blackmoor" falls to an invading force of northerners, the united bands of Nehron.
Their headgear was more elaborate, consisting of a steel helmet with a heavy nasal and surmounted with a spine of metal spikes. Horn plated brigandines protected them from wrist to waist, where a broad girdle bound a kilt of bearskin about their waists. Studded gambades protected their lower extremities, a circular shield (like Dunstan's) or one of trapezoidal form (borne by the two from Kimbry) graced the left arm, and the forearm and hand on the right were protected by steel gauntlets. Great capes of bearskin or like savage carnivora's served to both adorn and protect their backs. Their arms were massive flails and battleaxes, suited well to their strength. Brightly hued devices were painted upon their shields, but the northerners knew no heraldric laws, rather beasts or weird symbols were limned at the owner’s fancy.
I'm a big fan of "The Gnome Cache", and think that the world described would make a great setting for running "In Search of the Unknown". There's even a narrow pass between two steep hills, as mentioned in the Players' Background to the adventure.