Ridiculously simple, and yet strangely compelling! Maybe it's the music?
This game should suck because of the combat system. First of all, there is no combat system! Even in action RPGs like the Legend of Zelda (which is actually more of an action adventure then an RPG) the character has the ability to swing his sword (if he has a sword) in order to vanquish his enemies. Not in Ys Book I and II. The hero Adol Christin has a sword, but unlike anything I've ever played before he can't swing it, or at least there isn't a button for it if he could. Instead, as IGN bluntly puts it, Adol defeats his foes (or goons as the game calls them) by crashing into them "with his face!" I'm sure that's not how the developers would describe the combat system, but that's pretty much what it looks like on screen--Adol ramming goons with his face! In fact, with the exception of the boss fights, the only way a goon can even hurt you is if he some how manages to attack from behind.
Now your probably already thinking, wow, this game sucks! And you would be right if it wasn't for one factor that I forgot to mention: SPEED! This is one of the fastest action RPGs ever created, and on top of that you can select the options menu and increase the speed even more! At first it seems like this shouldn't make much of difference, but as you get deeper into the game you start appreciating the simplicity of the combat when you factor in the speed at which the game progress. The goons are almost flying at you, especially on Turbo, and wouldn't provide you enough time to brandish your sword anyway.
Now here is where it becomes satisfying: Because of the speed of the game, you are constantly leveling up. Unlike Phantasy Star and Final Fantasy, battles are incredibly quick making the "grinding periods" far less frustratrating then most RPGs. Instead of meticulously roaming the country side waiting for random battles until you get a high enough level to defeat the next boss, Adol can collide into his enemies continuously since enemies respawn the instant they are killed. With the game set on turbo you'll never find yourself "grinding" for the next boss fight for longer than 15 minutes, and constantly watching your exp. points rise in the corner of the screen is subtly satisfying.
You're probably sick of me justifying this crappy game with the "SPEED" factor, but combat isn't the only thing that is justified by speed. As stated before exploration consists almost entirely of mazes much like the dungeons in Phantasy Star II. However, once again the sheer speed at which you explore these mazes keeps the pace and the game interesting. It pretty much boils down to "look-and-move-on." There's really no waiting around on a certain template trying to solve a puzzle in order to move on to the next room, instead you're constantly on the go. If your still not sold on this game, hold on! I haven't mentioned the most incredible aspect of this otherwise crappy game: the sound.
The sound in Ys Book I and II is absolutely, beyond a shadow of a doubt, incredible!!! This is mostly due to the fact that the original game was on CD-ROM for the Turbo Grafx-16, which required an addition $399 hardware extension for playing the CD. The sheer cost of that addition hardware was mostly likely a factor in the eventual demise of the Turbo Grafx-16. The CD extension had some great games, but not at the cost of $400 plus. This is somewhat disappointing, because the Ys Book I and II's soundtrack is simply outstanding! Composed by Yuzo Koshiro himself, the tunes still haunt me long after I've turned the console off. Note: Not every tune is as memorable as some others, yet the soundtrack still contains some of the greatest music of the 16-bit era was well as in the history of video games.
I would like to hope that the music isn't the real contributing factor for why I liked the game, but I don't think it is. My reason being that given the choice, I would go right back to this game again and beat it a second time without question. I remember when I first played Ys Book I and II, I felt a since of disappointment at how silly the combat was, but what started off as being very odd quickly developed into an addiction. I am proud to say that I'm a junkie of Ys Book I and II.