The level editor was started because I needed levels to experiment with 3d graphics. Since I had experimented with using GtkRadiant as a level editor prior, I decided to use a brush-based CSG approach to level editing. Diverging from Quake's additive-based approach, my level editor begins with all matter in the world already present and you "carve" out the level with subtractive brushes. Additive brushes can then add world geometry, as in Radiant's case.
Since additive brushes function the same as in Radiant's editor, I can load Quake maps into my editor by first creating a large subtractive brush around the bounds of the map.
Unfortunately images can only give a vague idea of how the editor works. I have recorded a video demonstrating the editor in action: