Originally Posted by
Skies
Truth to be told, the dungeons are not quite required for the Main Quest aside from the first 3 dungeons, which serve as a tutorial on dungeons and gimmicks.
However, dungeons need to be unlocked through side-quests (which are ultra-short, it's normally an NPC in somewhere in town near wher eyou turn in a main quest that tells you "dungeon over there, go check it" you go over there and unlock the dungeon). Some of those (example: Halatali) are main-quest locked, you can't open them before certain points in the story.
Others (like Aurum Vale and Qarn), however, are level-locked and you need a minimum level before the quest for them appears. These quests tend to appear in towns, though.