Seems like you edited this. It is looking much much better than the original.

15 colors means you can only have 15 colors in your pallet. So each green counts as a 1 of those 15 colors.

I wanted to go too fast... I made it in only one time and I don't reflect enough before my inscription in this challenge.

It is too late for editing my participation? Can I delete this one to propose a better work?
I'm sorry, this world with its rules is new for me and the pixel art too.

I have an other question : What means "15 colors"?  For example : a dark and a light green is counted for one or two colors?

Is this finished?

There is so much room to get into pixelling here...a little bit of AA maybe a few more colors, some more shadeing or texture in the stone...seems like a wasted canvas as it is.