I love your style!
The cave-like thing looks amazing!

The technique is right, the style is cool,
but its unfinished, empty.
I think the underground walls should have texture, not just color.
The pebles that line them look boring imo, you have enough space there to make something more interesting of them.

I like it! I would probably look better if, as suggested by StickyTape, you made a smooth transition between grass and dirt, instead of a large line, then dirt...

Wow, very good the firt part.

I really like the sketchy look on the rocks and the edge outlines for shades. The grass is well rendered too. Wish the underground dirt wasn't a solid color though! The strong line that separates above ground from the underground is a bit harsh, but might not be so jarring if it transitioned into some dirt detailing.