Hagge Castle can be found in the northern kingdom of Aedirn. It is where a meeting was held by the rulers of the Northern Kingdoms, intended to take strategic decisions on the Nilfgaard offensive.

"Vizimir, Foltest, Demawend, Henselt and Meve, repeated the marshal. They met in a secret council in Hagge Castle on the Pontar. They conferred in privacy.

How symbolic, said the slender, black-haired man wearing an elk tunic marked with the imprints of armour and rust stains, without looking around. After all, it was at Hagge, not forty years ago, that Virfuril defeated Medell's armies, strengthened his control over the Pontar Valley and established today's borders between Aedirn and Temeria."
- pg.228, Blood of Elves (UK edition)