Invincible mobs in starter quests?

[TerraformX3]

This is something I don't get: On the starter island there are several quests which I cannot complete at level 7 because it has 'invincible' opponents. E.g. Mystery of the deserted castle and Archimage Rezo. Not to mention the group protecting the Kings camp (why are they hostile anyway?). One would expect mobs in the same countries are more or less of the same level, increasing slowly with each new country.

So: is the game not linear in advancement? Do we frequently get quests which we cannot complete before we grow much stronger?