@Zero I'd say it depends on what you didn't like about The Dragon's Trap.
In regards to Monster Boy: Monster Boy is much more modern. Death isn't as unforgiving as it is in Dragon's Trap. It's a much, much longer game (20 hours if you want to do everything), and feels more like a conventional Metroid game. It has an emphasis on environmental puzzles. For example, if you see a rope, you can likely burn it. This might drop a weight onto a button, allowing you to get through a previously locked door. It also holds onto Wonder Boy staples, like buying weapons and armor at shops.
The main benefit of Monster Boy compared to Dragon's Trap is that there's really no question about where you should be going or what you should be doing. Generally, the game always has a yellow dot on the map of where you need to go.
And DUH, it's SO simple!!
- Wonder Boy was originally Wonder Boy on the Master System and the arcade. When Westone went to port it to the NES, they couldn't keep the rights to Wonder Boy, so the game was
reskinned and retitled to Adventure Island. This is its own whole series.
- Wonder Boy (a linear, on rails platformer)
then had a sequel: Wonder Boy in Monster Land. This arcade/Master System game evolved with the idea of visiting shops to upgrade your gear. In a sense, it's like a very linear version of Zelda 2. Sidescrolling action. No experience, but you buy stuff from shops. Super, super unforgiving.
- THEN
there were TWO versions of Wonder Boy 3. The FIRST Wonder Boy 3 was similar to the original. An arcadey autoscrolling platformer/shmup.
The SECOND game was more like Wonder Boy in Monster Land. This was the Dragon's Trap. In fact, this game takes place IMMEDIATELY after Wonder Boy in Monster Land. It did the Rondo of Blood/Symphony of the Night thing long before those games did.
- Dragon's Trap had a couple ports. One to the TurboGrafx, titled Dragon's Curse, and another Master System game starring Monica, Brazil's premiere game mascot.
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Wonder Boy in Monster World (Sega Genesis) was the third game in the "Monster World" series, but the fourth game in the Wonder Boy series. You with me? This game might be pretty recognizable, as it was on the Sega Genesis. If you've ever watched My Life In Gaming on YouTube, you'll notice they use this game in almost every one of their videos.
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Monster World IV gets rid of the Wonder Boy branding altogether. It still has a lot in common with the older games (notably in that you make money to buy gear, and navigate dungeons).
And then YEARS LATER, they made Monster Boy, which is basically in the same Monster World universe. It has a lot of similarities with the Genesis game and Dragon's Trap, with many nods to Monster World IV. It's very much a Metroid style game, especially if you look at how it uses its map. It's kind of like the game that unifies everything that came before it. It's awesome!