When they had kindled a fire (περιαψαντων πυρ). Genitive absolute, first aorist active participle of περιαπτω, an old verb, but here only in the N.T. Kindle around, make a good fire that blazes all over. It was April and cool at night. The servants made the fire.

And had sat down together (κα συνκαθισαντων). Genitive absolute again. Note συν- (together), all had taken seats around the fire.

Peter sat in the midst of them (εκαθητο ο Πετρος μεσος αυτων). Imperfect tense, he was sitting, and note μεσος, nominative predicate adjective with the genitive, like John 1:26, good Greek idiom.