From A Wiki of Ice and Fire
and the location of the Last Hearth
Last Hearth, sometimes known as the Last Hearth, is the seat of House Umber in the North. It lies close to the Wall, beyond the Last River, and near the Bay of Seals. With the exception of the castles of the Night's Watch, Last Hearth is the northernmost major castle of the Seven Kingdoms.
A Game of Thrones
Lord Greatjon Umber and his son Smalljon Umber lead most of the Umber fighting men south alongside Robb Stark.
A Clash of Kings
In the absence of its lord, Last Hearth is administered by joint castellans, Greatjon's uncles Mors Umber and Hother Umber.
Mors and Hother request assistance from Bran Stark at Winterfell because the Umbers lack the manpower to harvest their land and repel wildlings encroaching from across the Bay of Seals. The castellans are tasked by Ser Rodrik Cassel to build longships made of Umber lumber with White Harbor assistance.
A Storm of Swords
Maester Aemon sends a raven to Last Hearth in a plea for help to defend Castle Black from wildlings.
A Dance with Dragons
Because Greatjon Umber is held prisoner at the Twins, Hother reluctantly supports Lord Roose Bolton as Warden of the North. Mors decides to support Stannis Baratheon as King of Westeros if he is given the skull of Mance Rayder and if Hother is pardoned for supporting Roose. It is unknown who holds Last Hearth in the brothers' absence.
The Winds of Winter
Theon Greyjoy tells Stannis that Mors took green boys to support Stannis, while Hother took greybeards to support Roose.
References and notes