Let me clarify something upfront—there's no single definitive manhwa titled 'Dungeon Defense.' However, the name strongly evokes the popular Korean light novel series 'Dungeon Defense' by Yoo Heonhwa, often referred to by its Korean title 'Dangeon Bangyeok.' Since manhwa adaptations are common, many readers search expecting one. The core plot, whether in novel or hypothetical adaptation form, revolves around a demon lord named Dantalian. After the human hero defeats the 72 demon lords, Dantalian is reincarnated into the body of his weakest, most cowardly past self at the very beginning of the game's storyline. He retains all his memories and intellect. The story is essentially a grand strategy masterpiece set in a dark fantasy world; Dantalian uses his future knowledge, ruthless political maneuvering, and psychological warfare to climb back to power, outsmarting heroes, empires, and rival demons. It's less about dungeon-building and more about Machiavellian conquest from a position of near-zero resources.
What's fascinating is how it subverts the typical power fantasy. The protagonist is physically weak but a genius tactician, and the narrative delves deeply into themes of nihilism, social Darwinism, and the nature of evil. The tone is grim, philosophical, and punctuated by shocking betrayals and reversals. If you're looking for a manhwa with similar vibes, 'The Great Mage Returns After 4000 Years' or 'The Regressed Demon Lord is Kind' sometimes scratch a similar itch of strategic resurgence, though they lack the original's particular brand of amoral, chess-like plotting.