Alright, so I got done play-testing and stuff, so I hope this is helpful!
-I really like the concept, it's one aspect I kind of wish SMBX had that came from SMW.
-One beef I had with this level was the fact I never knew how much time I had left (besides the 10-second countdown), and due to SMBX's capabilities, I don't really see a way for that to happen. Though I guess it can build suspense/paranoia from the sheer fact you have no idea how much time is left.
-At the end it seemed there was a slight red-koopa spam, I'd mix it up a bit just a tad more.
-I REALLY love the circle/platform graphics, I kind of wish you put some more details for aesthetics.
-Speaking of aesthetics, the Boom-Boom fight room seemed a bit bland, other than that it was fine. Nice use of SMB1 graphics.
-The weird crack graphics seen in the Boom-Boom room (see below) looks a tad ugly. Instead of using 1x1 pixels to draw the cracks, use 2x2 pixels instead. The same goes for the heavenly sequence arrows.
-You can bounce off the koopa as seen in the image to cheat your way around having to bring those mushroom blocks with you.
-The castle sequence kind of ends abruptly, you could probably extend the level a tad longer. Also, 150 seconds seems like way too much at the moment.
-I didn't like the heaven "light" graphic, The light itself should shine down in a cone style (as seen in the image below).
-The extra star wasn't too hard to get.
-The music fit with the level nicely.
-The heaven sequence plus the castle 'destroyed' graphic (when you're outside) were nice touches, haha.
-I died a few times, but that was due to me rushing to beat the level. Once I took my time, the level was fairly easy. I'm not sure where I would place this "world-wise," since I haven't got a clue where difficulty is at the moment.
Hope this is beneficial!