Glean, Ruminate, Buff the crap out of surrounding structures!
Sanctum is a defensive building which does not have an attack, but rather buffs the Damage% and Damage Reduction% of surrounding buildings by a significant amount and it's the place where you acquire Glyphs and Ruminate to unlock Glyph slots on Epic Heroes. See table below for detailed level information.
be sure to locate the Sanctum in range of your strongest defensive buildings to get the greatest effect.
If you are wondering what this actually does,well,someone even prepared not to even bother this building,i say can only say it makes your hero so OP with so low effort.Just do your dailies and you will get enough resources by a month to launch a Mega Gleaning party,literally.