Design
-The level honestly looks really bland, with no real variation on the design or anything to make it more interesting.
-Most of the enemy placement isn't really a challenge, and it doesn't look like it was well thought out.
-You also are using only spinies (and one or two thwomps) in the level, contributing to the boring, generic design (judging from the screen, of course). I suggest you vary the enemies a bit more, as said before.
Atmosphere
-The stone coins also look really weird, the normal coins would fit better, honestly.
-The 'Fortress' tileset don't fit well with the other tileset you are using.
-I'm also not really a fan of the white on the stars at the background.
Music
(btw, did you already think about what music you will use in this level?)
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Overall it looks okay, not really sure about the design, though.
Maybe it is better on the actual level, heh