The best way is to use a seperate cube for each wall, but you can Boolean partway through a surface, but in this case I would create seperate walls and Boolean holes right through them.
Darkon
Red bellows of flame have blackened my stones
Convulsing my frame and cracking my bones
Hell's dragons of steel who roar in their chains
Crawl into my caves to suck out my veins.....
-The Mountain P.F.M.