This.
Joey hits the nail on the head.
Words right out of my mouth.
Your level is very good for you're first level?
However, your Cliff's are tooo linear. You shouldn't necessarily make them random, but more cliffey?
Example:
The cliff's are not just straight right? They're constantly changing direction but that's what makes it awesome.
Go crazy on the ground. Honestly, in my opinion the outside of the level carries at least 64% of the overall levels look. You can make an outstanding building, but if there's just plain grass then it masks the wonder of the level.
In addition to what everyone else has said, your sand and water are behaving too linear. They don't follow a pattern.