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Welcome to the Runouwlution.

Your level had me very indecisive on whether to rate it 3 or 4 stars. Since this is your first map, I will tell you some important details to focus on from henceforth when designing maps for Super Mario 63.

Tile variation -- To achieve 5 stars on any level-- the levels which go above and beyond standards -- tile variation, the adding of similar-color/textured tiles within a large chunk of regular ones. In your level, the main tiles were castle walls. Adding splatters of 'broken brick' tiles always increases the decoration factor, without inducing too much lag.
Lag -- This strongly influences ratings, and is usually caused by a vast, open space with little, if any, level transitions. Also caused by too many items/enemies.
Use of items -- Don't forget, such as in a jungle level, to use decoration items, although not too liberally. Increases decoration
Difficulty -- Not too hard in a very long level, such as one that takes more than 9 minutes, only to end with a pitfall.
Open world feel -- Forget not that some noobs can get lost in an open space, considering that not only the end goal can be to the far right, but also Up, down, or far left. Arrows or signs can help in a big map.
Cutoff -- Not ending slopes with proper tiles, and strongly influences level rating.
Creative Quirks -- The use of rainbow bullet bills, or other ways to advance in the level, or decorate, will (at least from me) be taken into a consideration.

tl;dr: You have basic level competency, and I hope to see you (and everyone) continue to create levels. 12/20 ~ 3.2/5 ~ 3 stars.