My Doomsday Shelter Can Be Infinitely Upgraded

Celestial bodies fall to the ground, causing doomsday catastrophe. Extreme climate, radiation mutation, zombie craze, underground giant beasts, human cannibalism, alien colonization... Yang Chen was reborn and returned to the beginning of the apocalypse, with only 24 hours left before the disaster struck. Fortunately, he brought back the system developed in his previous life and opened his own doomsday refuge, which can be upgraded and evolved infinitely: Steel door → titanium alloy door → energy defense shield → space wall... Machine gun defense tower → Gatling position → Land short-range gun → Gauss rail gun... Biological laboratory → Circular ecological park → Military science fortress → Floating city... When everyone was struggling to survive in the face of the disaster, Yang Chen was already standing at the top of the food chain, looking down at this children's paradise-like apocalypse...