Start by Summoning the Fallen Valkyrie

The male protagonist Ian travels across time and becomes the last lord of an abandoned border territory. At the beginning, he faces a double-death situation of an siege by beasts and an internal rebellion. The main plot is highly focused on "survival and expansion", that is, by summoning a powerful heroine to solve the immediate disaster, and then using war to support the war, gradually regaining the lost territory, uncovering the divine conspiracy behind the erosion of the abyss, and finally establishing the immortal Winter Throne.