Coryoon starts off with an absurd premise. The eponymous hero, an adorable baby dragon, and a princess are hanging out together, when all of a sudden, a gigantic monster comes out of nowhere and turns the princess into a super-deformed version of herself. Coryoon goes out to exact vengeance on the monster and turn his princess back to a normal person. On the way, you fly through colorful levels, fight huge bosses, and eat lots and lots of fruit.
At first, the game seems like a fairly standard side-scrolling shoot-em-up. There isn't very much hazardous terrain to dodge and most enemies fly in assembled groups and patterns. Every so often a stork will fly by. They move pretty fast, but are quite huge, so they're easy to hit. After defeating one, it'll drop an egg, which will drop color-coded power-ups. Red is fire, which does a lot of damage, but doesn't have the best range. Blue is water, which functions like a laser, firing in a straight line and piercing through enemies. Yellow is a spread attack, shooting little lightning bolts in front of and behind. It does very low damage, but can still be useful if you know how to use it well.
In addition to gaining power-ups through eggs, Coryoon can collect spades, hearts, clubs, and diamonds for other special powers. Spades give you a screen-clearing bomb. Hearts give you tiny dragon helpers, clubs are basically fruit magnets that make fruit come to you instead of simply falling, and diamonds shrink your sprite and hitbox. The mini dragons are kind of underwhelming, as they run away once you reach a boss, but otherwise all the powers are pretty cool. You can also kill giant chicken enemies to gain power-ups.
Coryoon also has some standoint boss designs. Each level has two bosses, one at the halfway point and another at the end. Most bosses have multiple forms and a variety of interesting attacks. One of the cooler bosses involves fighting a monster's head surrounded by fire. After defeating it, its full body appears and it gets furious. Another fun battle is against a giant turtle with crystals growing out of its back. It has a number of different attacks, most of which are very bizarre.
Coryoon's gameplay may seem similar to anyone familiar with the first Air Zonk game, which mysteriously took on a different feel in its sequel. This is easily explained by the fact that Air Zonk and Coryoon shared staff: Yashiharu Takaoka was the designer and main programmer in the first Air Zonk and had the same duties for Coryoon, hence why the two feel so similar. Meanwhile Hishashi Matsushita and Daisuke Morishima worked on the music for both Air Zonk and Coryoon. While the games don't have much to do with each other, as they were developed at different companies, they still retain a similar style.
Coryoon is one of the most graphically impressive games on a Hu-card. It features many planes of scrolling, huge bright-colored sprites, and it moves extremely fast. It also fits a lot of sprites on screen at once. Whenever an enemy is killed it drops fruit. Fruit comes in many different types and sizes, and are all worth points. The game is quite forgiving, too as it practically throws extra lives at you like candy on Halloween, and you don't restart at a checkpoint, making it one of the easier shmups on the PC Engine.
There are a number of cool extra features. If you beat a level you unlock that level in stage select, which starts you on the selected stage instead of the beginning. You can also turn off the fruit, which will make your score lower. On the upside, you're much less likely to have a seizure during some of the game's more hectic parts. Slowdown is practically nonexistent, even with dozens upon dozens of sprites on screen moving at lightning fast speeds.
Coryoon was a fairly late release on a Hu-card. By 1991 the CD add on had been released, so it was probably skipped over. It probably didn't help that many of Naxat's games never left Japan. Coryoon was sadly not an exception. As a result, as of 2011 it commands a price of over $100 (USD) for just for the HU card itself.