Monster Tale is "evocative in a good way".
As Gabe says, the difference between an homage and a rip-off is whether or not I like it. Monster Tale is an homage to Castlevania and Metroid, plus a little Pokemon and Monster World. I liked it. It reminded me most of Symphony of the Night and Metroid Fusion.
Castlevania's levels are meant to be explored. They want to be explored. They're filled with the beautiful tension of enemies that mean to stop you (in the most ridiculous or undead way possible). With every new screen, you get to see a little more of the craziness that Dracula decided to pack his castle with. With every new screen, you may run out of health before finding the next save point. Symphony of the Night finds the best level of difficulty: every time the path forked, I stopped. I tried to guess which path led to a save point. And I debated whether to go back and save my exploration up to that point. It's no mistake that there's a huge exploration percent displayed on the map screen.
Compared to Castlevania, Metroid Fusion seems bossy. It's always telling you where to go, because Nintendo (unlike Konami) must have written down a policy early in the life of the GBA that games must be playable in 5-minute increments. That means a game that is not quite as mysterious as Super Metroid or Metroid Prime, both of which have the Valve Property: wandering around lost will get you where need to go--but it may take more than five minutes, and you may have a chance to absorb your surroundings first.
Like Fusion, Monster Tale sacrifices this organic mystery to 5-minute playtime, with the additional handicap that its target audience appears to be 5-year-olds. Not that a well-crafted Metroidvania can't get by without a good story (see: Portrait of Ruin, or, to a lesser extent, every Castlevania <i>ever</i>). Actually, Monster Tale is a lot like Portrait of Ruin in that its overall structure is a disconnected Mario-3-esque Theme World design. At least they aren't the literal Mario 3 Themes--instead of Desert/Water/Air/Ice we get Halloween/Runaway Train/Seaside/Club(?). I guess these Theme Worlds are places the authors think kids would enjoy, but like Portrait of Ruin, the Themes are skin deep coverings over level design that is actually a bit bland. A lot bland, if I'm being honest.
What saves Monster Tale is the difficulty. Although the target audience appears to be kids, the difficulty is aimed at, um, slightly older kids. Really, we were playing (and beating (some of us (not me))) games this hard by age 10 at least--Mario 3 is easily twice as hard. By modern standards, though, this is a hard game. I had to play four of the five bosses multiple times, learning patterns, fiddling with my pet to get the right combination of skills, balancing offence and defence. That's above average difficulty in today's world, especially for kids.
So parts of the world are Genuinely Dangerous, and health drops don't happen that often; I knew they had re-captured the Symphony of the Night feeling when I stopped for an internal debate at every fork in the road--which path seemed more likely to lead to a save point? Which would wear me down, kill me and waste my hours (5 minutes) of exploration? I could ignore the bland levels and haphazard enemy placement because I was having fun beating enemies and getting beaten. The enemy-juggling mini-game contributed to that, but it was mostly good old-fashioned meaty Castlevania Action.
Last, and actually pretty close to least, there's that Extra RPG Spice, permeating the gameplay like MSG whenever the actual flavour gets too boring. Here, the Spice is Pokemon, or something like it. Sure, it's manipulative, but watching your pet level gives the mind something to do when the body is backtracking through the lower-level parts of the world. Anyway, it's not completely straightforward--after I finished the game I visited gamefaqs and found out why some forms of pet never became available; the reason is that they have specific conditions besides "level up like crazy". So that's nice. It wasn't a lazy copout; they actually cared. In fact, most of the "innovation" in the game revolves around the pet, but it turns out most of the "fun" comes from core game copied from Metroid and Castlevania. Take that for what it's worth.