Ah, I killed him at tier 2 without building a wind shelter. I think he was over tier 5. Playing at default settings. So overlord is a set difficulty non-scaling? Would be nice to have more of the game very very hard for Tier 1, give 'leveling' a meaning
The overlord is base tier 5, any enemies in a wind storm get +2 Tiers, so he'd probably have been at Tier 7. That means he was doing 6 times the damage he should be. Default difficulty settings are, in my opinion, ludicrously low, I'd say change to Hero difficulty, see how you go, and adjust the difficulty from there.
As to the other assertions, there are certain areas where the tier is higher (The Deep, deeper caves), but doing it generally would require a complete philosophy change for much of the way the game is done, currently you choose your spells and then go to the required areas to get resources, if most of those resources were locked away in high tier areas then you are essentially funnelled into using whatever limited selection of spells you could get from the low tier areas.