hidden level is a level that becomes active when you get to glowing frame most of the levels before an epic boss. For the first boss you need all but the last two to be glowing for the hidden level to appear.
i must be doing something wrong - i barely manage to get past the last waves of mobs on the first epic battle because it takes a lot of time to get the right colour of gems. And as to the hidden level..i can't even get past the first 10 waves
some of the items have wierd description: "Reduces direct damage by 5. Increases direct damage by 10." So it increases.. or not. And what ?:) *i did understand what it does*
 "GemCraft"){kind=icon}
 "Ring Pass Not"){kind=icon}
 "Talesworth Arena: Death Watch"){kind=icon}
 "Caravaneer"){kind=icon}