Hollow Hill

The Hollow Hill is a sprawling underground complex of caves and tunnels. It is adopted by the Brotherhood Without Banners, and used as their base of operations during the War of the Five Kings.

Season 3
Arya, Gendry, and Sandor Clegane are taken to the Hollow Hill, following the latter's capture by the Brotherhood Without Banners. There, the trio are introduced to the Brotherhood's commander: Lord Beric Dondarrion, who sentences Clegane to trial by combat.

In the books
In the A Song of Ice and Fire novels, the Hollow Hill also serves as the headquarters of the Brotherhood Without Banners.