To be honest, this was worse than the last level of yours I played.
Firstly, cutoff.
Secondly, Boswer's Fortress is a really generic name. Try for something more original.
Thirdly, the platforms to the left in the first section are a bit over rewarding. Just the leaf would have been fine.
Fourthly, the first section is bland. You just go from platform to platform with some koopas here and there. Maybe some Paratroopas, other enemies, or other obstacles would have helped?
Fifthly, your powerup to enemy ratio is really off. You give a lot of powerups and there aren't many enemies.
Sixthly, your coin placement is bad. It's just on the ground. It should be in the air, y'know?
Seventhly, the vine in Section 3 should be growing out of a block, for aesthetics sake.
Eigthly, the first part of Section 4 is bland and boring.
Ninethly, the orange vine should be going up until the top of the screen like the green vine was, or should be stopped by a block. Not like this!
Tenthly, why are the bullet bills standing on SMB3 Tube-NPC-block whatever-you-call them instead of on the regular bullet bill stands in the Misc. section of SMB3?
Eleventhly, why does the sign in Section 6 tell you to go left instead of right?
Twelvethly, the checkpoint is placed too early.
Overall, this level is pretty bad. It wasn't very enjoyable, and there wasn't anything special about it. It had plenty of flaws. I say this should go in The Trash Can or a low Average.
You should figure out everything in SMBX, including all the blocks/backgrounds/npcs and what they do, Layers, Events, and Graphic Swaps. YouTube and the forums will help you there. Then, look at levels from The Best and see what they do and why there are good.
Remember, quality > quantity.