Procedural terrain- of the 2D circular variety.

Hey all, I've been toying around with this idea I had for far too long.

What I'm trying to achieve is similar to the terrain shown in this Voxels for unity tutorial.

But mapping that lovely perlin noise terrain to a circular shape.

A 2D byte array holds the values of each cell, and I instantiate a sprite in every cell with a value. The tutorial uses a mesh because it's better- I'm using sprites because I'm lazy and wanted to see if I could make it work. But I digress.

So far I've gotten this- by cycling through the degrees of the circle, using noise to determine the extents, and attempting(unsuccessfully) to fill in by checking every integer point along the radius for a cell with no value. The easy slopes of the "bottom up" terrain aren't present in what I get with this method.

I'd appreciate any pointers you could give me! Thanks.