Many Wonder Boy fans consider Wonder Boy in Monster Land to be the most difficult game in the long-running series. That being said, this is the first Wonder Boy game I've ever played, and I have to say, while I have no basis of comparison, I can certainly say this game is indeed rather difficult. It is certainly not a controller-smashing, scream-at-the-TV affair, but it's certainly not the easiest 2D platformer, either. 12 levels doesn't sound like much, but with no ability to save or use a password, and few opportunities to get extra lives to add to the two you start with, the difficulty gets bump up a few notches. Also, don't forget that you have a time limit for making it through each level, and if you bust your limit (easier than it sounds) you will lose health for it.
You start your quest by going into the first door you find, and a guy explains to you the standard line about how you have to save the world and such, gives you a sword and sends you on your way. The interesting thing about this game is the fact that isn't just a normal 2D sidescroller, it actually incorporates some role-playing elements as well. Wonder Boy will not be using the same sword by the fourth level as he was given in the first. He will no longer be barefooted with no armor either. As you progress through the game, you gain money by defeating enemies, and by walking over the right spot of ground. You can then use this money to buy upgrades for your character, which are nicely laid out for you on the pause screen. That particular element of the game is the biggest part that kept me coming back for more. It's a lot of fun to try new combinations of varying levels of armor, shields, footwear, bombs, fireballs and so on to see which ones will work best.
The game itself plays pretty nicely, with relatively responsive controls, though they are not as precise as they should be considering how unforgiving some areas can be. You have to hit your enemies with the sword just right in order to hurt them, and when you do hit them, they freeze for a second and you can't hurt them again until they unfreeze. Usually when they do that they walk right into you and take some health from you, so make sure you take a few steps back after smacking an enemy so they can't revive and immediately counter you. If you time it right you can keep hitting them the second they revive until they are dead.
The graphics are very bright and colorful, and they fit the upbeat music very nicely. There is minimum flicker, and a pretty good variety of enemies, though later in the game you will notice some familiar faces rendered in a different color and magically stronger, but no big deal. The music is the usual happy adventure sound, and I really enjoyed it. It felt right with this game, and the sound effects were also pretty good, albeit simplistic.
The last thing left to cover are the boss fights, found at the end of each level. Some are much easier than others, but like most games coming from this era you can dramatically reduce the difficulty level by memorizing their patterns. This is especially true when you encounter the boss that will die simply if you answer his question right. If you get it wrong then good luck beating him the normal way, if you're like me you'll need it.
On the whole, I would say this is easily the best Master System game I have played. Bear in mind it is also probably the sixth or seventh I've played, and I haven't played Phantasy Star on it yet, so please keep that qualifier in mind. Nevertheless, I recommend this game for Master System owners and fans of 2D platformers in general. You'll easily get a few hours of light-hearted enjoyment out of it, and what more do you really need?