Dick Adams wrote:

While some crude forms of distillation may have existed
before 1200, the stills probably couldn't get the ABV much
higher than 25% to 30%. We know that, with tender loving
care, you can get fermentations without distillation to
from 20% to 25% ABV.

And you can get higher concentrations by freezing
the crude prep and fishing out the ice. When
the crude prep is apple wine, the result is called
applejack. In northern Europe, this may have been
discovered accidentally when barrels were stored
where they would freeze in the winter (which is
pretty much anywhere in medieval northern Europe).

A few years ago, when I researched this on the web,
it quickly became apparent from reports of hobbyist
applejack-makers that you don't get a very good
result. It concentrates the alcohol, but it also
concentrates the fusel oil. Distillation gives you
a much higher quality product.