Hi Sofaspartan,
the last level 5-1 was most fun.
The gameplay of the levels really give some super mario vibes.
Inside the water level having "to collect all keys"... after reaching the end felt a bit annoying... think it would be good to have clue right from the start... that some keys are needed to progress... like having to go back and search for all six and loose them all after death, fealt a bit to cumbersome.
Inside the air level: dying from one hit felt also relative punishing.
For me there is something a little bit off about the way the enemies moves (maybe the move to fast or something)... but in mario everything about hitting and landing on enemies feels somehow smoother.
What i definetely enjoy most in "normal mario" levels ... are secrets "which are a bit hidden" ... something which you might overlook in first gameplay but there is somehow something unexpected leading to a secret room (maybe with a marker on the map telling me that here is "something extra to find".
The gameplay overall feels quite solid
Me personal I also enjoy some kind of trampolins and something i noticed maybe the apples look something "i want to pick from the trees" and I didn't figure out "how I can destroy the walls (which where for example inside the first level and get the points behind them).
For me there is something off with the ai generated music... can't put my finger on it, but I needed sometime to get used to it.
Have fun :)