There's a basic element of video game design that I feel just works if you apply it when making a game: design a fun concept, then use that concept for all it's worth. And that's what Konami did here.
Your basic abilities, run, jump, shoot your sword, boost forward with jetpack, and spin, never change. What changes is the level design and how it uses these elements. There's a new gimmick or three in every level, and you never know what's going to happen next.
One section has you switching between the foreground and background of a waterfall to jump from platform to platform, with the background elements being harder to see behind it. Another has you dealing with rising and falling lava which happens to reflect what's above it, minus the foremost foreground elements. In that section, once you can't see your character because he's obscured by crystals blocking your view, you can simply wait for the lava to rise and then judge your jumping distances based on the reflection. Later in that same level, you can ride an automated mech across lava, but have to jump off when it starts walking underneath a low-hanging spiked ceiling, only to get on again when the ceiling is no longer a threat and the lava returns.
These segments last less than a minute in many cases, so no gimmick wears out its welcome. And since the play mechanics don't change, it doesn't feel like the game suddenly swaps genres on you. Instead, you're playing a platformer with insanely creative level design.
The game also moves FAST. Very fast. The movement speed of your character, and all the other elements, is unexpected but it keeps the action flowing quickly. This game can get intense.
Which leads to my one gripe: the difficulty levels. "Easy" in the US version is equivalent to "Hard" in the Japanese version, and "Normal" is equivalent to "Very Hard". This is actually a renaming, not a change, but it gives an idea of just how hard this game is. It can be brutal at times on any difficulty higher than the lowest, and you need fast reflexes to survive.
I don't care for the sequels much, as I feel they lost the elements that made this game fun, as well as its catchy music and goofy charm. But the original Rocket Knight Adventures
is simply a well-made action game.