Protect Ryugu Castle!

Editorial Review

★4.3 / 5 — henhenta editorial rating (our own assessment, not the DLsite official user score)

Protect Ryugu Castle! occupies a rare niche in the simulation space: a tower-defense game where your defensive line is a living, autonomous ecosystem rather than fixed structures or units you command. The appeal lies entirely in emergent gameplay—you dig, the creatures spawn and self-organize according to their predator-prey relationships, and their territorial behavior becomes your fortress.

What distinguishes this work is its commitment to ecological simulation as core mechanic rather than flavoring. The eight sea-creature types (kelp, sea urchin, octopus, firefly squid, sea hare, ningyo, umibouzu, skeleton) form a hierarchical food chain, and which species spawn depends on soil composition—nutrient-rich earth yields herbivores, while magically-dense soil produces the exotic and dangerous. You have zero direct unit control; instead, tactical depth emerges from how you carve the terrain itself, channeling creatures into narrow passages or herding them into open chambers. The five invader archetypes (fisherman, spearman, monk, armored warrior, turtle maiden) each present distinct threat patterns, adding asymmetrical challenge. The skeleton-conversion mechanic—defeating enemies and resurrecting their bones as allied units—provides satisfying long-term progression.

The feature set rounds out solid: eight player-purchasable skills (ranging from targeted healing to earthquake AOE effects) add direct agency without breaking the ecosystem-first design philosophy, an eight-stage campaign with post-game hard mode extends content, and sandbox mode lets players experiment freely. Bilingual support and extensive control remapping (mouse, keyboard, gamepad with granular customization) signal thoughtful accessibility.

This appeals specifically to players who prize systems-driven gameplay and emergent narratives over narrative or graphical polish—those who’d rather debug an ecosystem than execute a combat combo.

A clever, mechanically-focused alternative to conventional tower defense that rewards patience and ecological thinking.