The western foothills and flanks of the mountains are home to Hill tribes, warrior clans of varying size but unmitigated ferocity against the civilized inhabitants of the region.
In the books
In the A Song of Ice and Fire novels, the Mountains of the Moon are tall and forbidding, among the tallest peaks in all of Westeros. The largest pass through the mountains is guarded by a fortress known as the Bloody Gate, while the Eyrie, the seat of the Arryns, is built on the shoulder of the Giant's Lance, the tallest mountain in the region.
Due to their superior knowledge of the terrain, attempts by either the river lords or the valemen to wipe out the Hill tribes have failed. However, they lack the knowledge and technology needed to successfully prosecute siege warfare against the Vale of Arryn, which they regard as their true birthright.
Mountains of the Moon · The Fingers