How low can you go?

Hari & Ows bought me a bottle of Viking Mead, made in Jorvik, to an old recipe. Katie commented that her bee-keeping collective had made some mead last year, and drank some after this year's honey harvest. She said it was like rocket fuel. This led me on to wondering what max % alcohol could be achieved by just brewing alone. I thought it was about 15%, similar in strength to this mead, Port or Sherry. I've even told kids this in lessons. 

It turns out that I was mostly correct, with a few caveats, such as special yeasts, and slow adding of extra sugar. Now, that's a challenge!