I've only ever played dual wielding characters, just because, well, I like it, and I haven't created that many different characters anyways. I did for a very short amount of time use some two handed weapons, but that didn't last.
The point - when it comes to melee, my characters have been strictly offensive.
I have been adding Heavy Armor to my builds because I am currently in a phase where I like playing tank style characters. I've been playing on Master difficulty because my first run through on Adept was, well, way too easy.
But I've noticed that even with a bunch of good for my level Heavy Armor, as well as improved via Smithing to even Exquisite levels, as well as an Alteration shield spell on top of that, there is not much in the way of tanking for my characters. I get one, two, or 3 hit killed by pretty much any mob I go against (unless they are the pansy level bandit, forsworn, etc... i.e. the low level spawns).
Now, I do know that I am not as good at defense as I could be. My Heavy Armor perks go into Juggernaut only, and I am using dual wield, meaning I don't have a shield or even a two handed sword to block and absorb damage with. And as a result, my Heavy Armor skill doesn't raise fast, because instead of being the one to take the hits and raise my skill, I sit back casting spells until my follower and zombies pull aggro, then I swoop in and finish off the mobs with my dual wield damage.
My questions is - am I failing at being able to tank because I'm playing on Master difficulty, and the damage output of mobs is simply too much to ever be able to tank? Or am I failing at being able to tank because having a shield makes that much of an impact in absorbing damage?

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I didn't seem to catch what level are you anyway.
). And don't be fooled- mudcrabs indeed dish out some high damage, at least the ones with bigger shells. And I'm not even joking, I had many characters killed that were lower than level 5 that were attacked by 3 or more mudcrabs at a time on master.