Holes cut in the stone of the wall and filled with stained glass are called plate tracery, elaborated based on stone blocks or "plates" that are part of the muraría structure, in order to create a perforated pattern. It arises at the end of the XII century and the beginning of the XIII, before the need to add a decorative motif that would not leave bare the ashlar area that was free when two or more lancets were placed next to each other under an arched drip molding.
I think in this quote, Zeus is referring to the fire as the "evil thing in which they may take pleasure to their gut," because he refers to it right before he uses the word evil to describe it.
