If I didn't miss something essential, you can't achieve a concave form with gradients, because the level blocks are always divided into convex blocks.
Columns are no problem because they are convex.
You can use a model for that wall.

http://www.puppenheim.org/
http://www.wuerfels-welt.de
Old Game Tutorial: http://www.puppenheim.org/MITutorial.zip
Free Game Editor: http://puppenheim.org/AlienalsWeihnachtsmann.zip
http://www.puppenheim.org/blender_starting_main_settings.txt