Gekizumi Dungeon Desu ga "Skill: Hanshoku" de Gyakuten Shitai to Omoimasu

Reborn as a dungeon master in another world, Akira Hanamura quickly constructs a comfortable dungeon within his first 30 minutes in this new realm. However, he soon realizes his creation is a "zero-defense" setup — vulnerable to the constant influx of adventurers and, if conquered, spells instant death for him.