13 Bosses reside in the 13 castles. They increase in difficulty as each new boss is challenged. These bosses can be repeatedly challeneged to collect their loot (known as farming). in general, the higher level you are, the better it is to stay in the current area, and adjacent levels.
Quests show up on your map as a black skull and crossbones icon. These fights are generally more difficult, but provide a better reward and experience bonus.
The Mysterious Chasm is located on the middle-right of the map. This is a high level area, where increased money and loot is earned. Multiple waves of enemies (including lower level bosses) may spawn. Level ranges from 51-81.
Survival mode can be activated by traveling to the bottom-right of the map, and walking onto the blue banner. This mode allows you to challenge unlimited waves of enemies, including bosses. As you progress through each wave you have the chance to unlock better gear. You can find a list of survival mode gear on the armor page.