I don't understand your question, assuming there's a question in there. There's nothing stopping you from going to the Dark Forest early, but there are higher level enemies in the dark forest then you'll find in castle part of the game, and there are some quests in the dark forest you can't start until you've done the castle.
There are quests in the Verdistis sewers? I can only think of one, and an unofficial one at that. There are a couple of quests in the Rivertown Sewers, though.
