This mod was certainly more challenging than Cave Story. Some parts (the [edit]ing Tunnel) were a little too challenging in my opinion, but maybe I'm just a n00b. And other parts, like that building in the middle of Grassy Hills, and the Sandcrocs, were too repetitious. I felt that the Ancient Cave was too big and dull for the amount of times you have to go across it. Swatting away critters, mushrooms and bats for that long got tedious. I think the easiest way to make Ancient Cave more enjoyable is to shorten it, and but some secret bonuses in. I kept thinking to myself, "A place this must must have something to explore." And it was odd thinking of Jack as a different person, but I know how tough sprites are to make. Perhaps he should have been an unused sprite.
I hope Curly gets a new weapon soon; it would sure spice things up (even more).
One more minor complaint before I get to the praise: Quote's air tank. I was delighted when I had to go steal it, but mostly because I assumed Curly would be using it. It would be a WONDERFUL twist if Curly got Quote's air tank back from the Greasemonkey later, instead of finding her own.
Now for ego inflation: I think the story was well-thought out (aside from the air tank and the random Gaudis), and I can't wait to see what role that commander plays in setting up the story of Cave Story (redundant, I know). When the sand caved out from under Curly in the Sand Zone, it totally took me by surprise. I was all like "Aw yeah, time to rape some Omega!" and then you were like, "THAT'S WHAT YOU THINK!"
Speaking of Omega, it was the same fight as in Cave Story. I think you should change it somehow; just giving it more health would be enough. And speaking of bosses, I hope I don't have to fight too many more captains. You're fought Curly, you've fought them all.
Original art always pleases me. I smiled when Curly's angry face came up, and the robot enemies in the tower were awesome, even if their dying animation looked a little weird. And the assassins looked cool.
I know I put a lot of criticism into here, but I really like this mod. Back story for the win. Just one question: why does the Sand Zone change so much? Is it because the Demon Crown is making it moar evil really fast? Because biospheres don't change that rapidly.
Wait, biospheres? You're fighting skeletons. Never mind.