Northwall Mountains

Snow-capped peaks rising high over the Savage Lands, passes frequently blocked, the Northwall Mountains serve as a nearly-solid barrier between the northwest part of Lasair and The Great Plateau. The mountains are riddled with tunnels, however, thanks to the Ratlings who are native to the region, building their warrens in and under the mountains.

Geography

The Northwall Mountains stand astride the border between the Savage Lands and The Great Plateau, with a small bit protruding past the Titan Wall into the City-States Region. The mountain range is the tallest on Lasair, towering over the Savage Lands below. Their elevation and high latitude ensure that the highest reaches are covered in snow year-round, with the snow reaching low into most potential passes through most of the year. 
The mountains are less high relative to the Great Plateau, thanks to the latter's higher elevation, but they still present a significant rain shadow which causes the northwestern portion of the plateau to be drier than the rest of the plateau. When combined with the potential for Ratling raids, this has led to that region of the plateau having much less farmland than the areas closer to the rivers.

Ratling Tunnels

The Ratlings of the Savage Lands make their homes primarily within the Northwall Mountains. Over centuries they have built a network of tunnels and warrens throughout the range, using them to move rapidly up and down the region - as well as sometimes crossing up to the Plateau or even bypassing the Titan Wall to raid the City-States. 
The Ratlings have occasionally dug deep enough in the mountains to have broken through into ancient Giant ruins - but to them they're simply rooms that already exist. Most of these spaces are simply underground halls with nothing of power or value, but it certainly is possible that the ratlings could have found their way into spaces that others with enough knowledge could exploit.