The Siege Engine

When the world ended, humanity hid. Jaxen Vance built a wall. Fifty years after the Convergence turned Earth into a monster-infested wasteland, survival belongs to whoever has the strongest walls and the biggest army — and Jaxen has neither. What he has, after his settlement is slaughtered and he's left for dead in the ruins, is a glowing crystal and a System class nobody has seen in a decade: Aegis Warden. While everyone else levels up by fighting, Jaxen levels up by building, turning scrap metal into plasma turrets, dirt into acid moats, and a ruined shack into a Tier 5 Bastion that grows stronger with every attack it survives. But a warlord wants his Core, a Broodmother is organizing a horde of ten thousand, and the fortress that should be more than enough is about to be tested by an apex predator that has been evolving specifically to break it.