@JeremyECrawford Can Tortles using the 17 AC from their shells benefit from the defense fighting style?
— Michael Socci (@MichaelSocci) September 17, 2017
The Defense fighting style requires you to be wearing armor. Having natural armor doesn't count as wearing armor. #DnD https://t.co/DY2260KgoS
— Jeremy Crawford (@JeremyECrawford) September 17, 2017
Does wielding a shield count as wearing armor in this case? No.
— Jeremy Crawford (@JeremyECrawford) September 18, 2017
You could however use the mariner fighting style which requires to just not be wearing heavy armour
By RAW, there’s an even easier solution. Just wear armor.
The tortle’s Natural Armor says “you gain no benefit from wearing armor”, but it does not actually prohibit you from wearing armor.
The only requirement of the Defense fighting style is that you wear armor – not that you benefit from it. So just buy some leather or hide armor for 10 gold (padded is only 5 gold, but imposes disadvantage).
The DM might argue that you need to have it specially modified to fit you, since “due to your shell and the shape of your body, you are ill-suited to wearing armor”, but at worst this should bump the price up to 40 gold, the price of barding, which is what this would effectively be.
So for a price of 40 gold, 20 or 24 extra pounds of gear weight, and the investment of a fighting style, you can grab yourself an extra 1 AC. Combine that with a shield for 20 AC, without stealth disadvantage.