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Just look at those lovingly crafted, square edged graphics. Even the sound and music play like those limited channel, simplified beeps that marked the era. The game plays very smoothly even with just a keyboard but if you use a game pad then it feels even more like those classic titles.
In Doukutsu you play a guy who wakes up in a cave and saying more than that would spoil things. Besides, that's all of the information you have to start the game with and what would be the fun in an exploration game if someone hands you the info. You'll proceed to explore the cave, shoot up monsters, fight giant bosses, solve platforming based puzzles, and search for the power ups that will give you access to the next area.
Besides being a blast of complete nostalgia Doukutsu is a lot of fun. It's challenging but not so challenging that it made me frustrated. You'll encounter a tough boss fight about ever ten minutes of play which means you'll encounter a lot of variety in enemies as you get through the game. The environments are particularly large which means you'll spend quite a bit of your time player trying to find your way through them. There are plenty of bonuses hidden in the game to reward the deeper exploration, too.
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There is one more issue with Doukutsu and that is the game text was originally in Japanese. There is a patch to translate the game to English that if you do not read Japanese you will want to install before you play the game. Sadly the dedication to duplicate that 8-bit gaming was not replicated by the translation team; the conversations are coherent and not filled with Engrish. Would it have killed them to tell me, "A winner is you!"?
Doukutsu may be short but it's entertaining for every bit of that game and digging into the cave is fun. The whole thing is like being transported back to 1987. It's well worth the price tag and your time.