Look up 3D projection. Don't be scared of the matrices, you can either trust that they work or use lots of trig.
Also, why not look at the source for PotC: https://github.com/skeeto/Prelude-of-the-Chambered
And from the /r/gamedev sidebar: http://euclideanspace.com/
Very simple, likely crappy method off the top of my head would be to project all your geometry to 2D, and flood-fill in the spaces between points with solid color, starting with the shapes that are farthest away. That should get you started.
Edit: Note that what I just said is applicable for J2D or similar DIY rendering. If you are using lwjgl or similar, just use that.
You can use Decals or ShapeRenderer in libGDX for this kind of thing: http://www.mediafire.com/download/5ktjwqs6s3bjyjv/Tiles.jar
(From a topic I made a while ago, WASD to 'walk')