Classic indie game with a story and art-style reminiscent of japanese "Spirited Away-type" anime fantasy.
Cave Story is a great platform indie game that is up there on the same level as Super Mario Bros. and Metroid, in my opinion. It is so good because of it's unique story that keeps you interested, the variation in the environments across stages, and the mystical way the plot slowly unravels character details.
The game excels its contemporaries in emotional depth, because of the character developments. It takes advantage of multiple branching paths from your characters choices, which can lead to different endings that can impact your friends end-game fate.
This depth is what has given Cave Story cult-like status. It allows the game to be enjoyed by all ages, in the same way that Pixar finds success with Toy Story. There are little things that adults may catch, but the core gameplay will not be disrupted if you miss these things.
Now onto combat, which plays a big role in this game:
You have multiple weapons, which can grow from level 1 to 3. In order to progress up levels, you must collect bells which are dropped from slain enemies. However whenever you take damage, your level-up metre gets shaved down, and you can lose levels by taking more damage then you can recover via dropped bells. This technique will keep you on your toes at all times, no matter how full your life gauge is.
The weapons are fairly standard. You start with a gun that shoots energy at a medium range, and you can later find a bubble gun, a throwing sword, a 'fire flower' type Super Mario weapon, among others. Branching choices force you to play multiple times in order to see all possible weapon choices.
Now this game is not without its faults, and the following reason are why this game scored only 8.5 and not a 9 or a 10.
First of all, the objectives are unclear. Most of the time you can probably guess at what you're supposed to do, and where you're supposed to go next. But I am willing to bet that at least once during your first play-through you will be running back and forth between all the stages with no clue how to advance the plot. Online walkthroughs can be confusing because of the different possible branching choices, but youtube videos will likely help get unstuck.
Secondly, the interaction sequences in order to progress properly can be confusing and poorly designed. Sometimes if you interact with B before A, you will miss a key plot choice, because you must interact with A and then B, and then A again in order to get the correct prompts. And this is not obvious is the slightest. This can be frustrating if you expect to save all your friends and get the good ending on your first playthrough without cheating. Thankfully it is not gamebreakingly frustrating, because the game is fun and this creates some replay value.
I highly recommend reading into this game and watching the youtube trailer, if you are interested. It's a great game and in my mind definitely worth picking up!