What does Mark 14:66 mean?

Mark forgoes his habit of alternating stories and fully separates Peter's denial and Jesus' trial, despite the two events occurring simultaneously (Mark 14:54). John sets the stage further. John knows either the high priest, Caiaphas, or the high priest-emeritus, Annas (or both), and so follows the guards into a deeper courtyard. Some archaeologists think Caiaphas' house has been found. It sits on a hill and has several levels. Although the Gospels describe Peter's movements using different words, the complexity of the architecture doesn't allow for a simple description.

Peter follows John, but stays outside the door. When John realizes Peter is not with him, he tells the servant girl at the door to let Peter in. The girl complies, but recognizes Peter as one of Jesus' followers. Peter denies her accusation and joins the servants and guards at the fire (John 18:15–18). Any tension with the other Gospels here is not contradictory; it could be that the girl at the door doesn't accuse Peter until he reaches the fire (Mark 14:67).

Alone, perhaps among some of the very servants who arrested Jesus, Peter is reluctant to identify himself with the man now on trial. On the way to the Mount of Olives, Peter was brash enough to state that even if all the other disciples fell away, he would never betray Jesus (Mark 14:31). In the garden of Gethsemane, he drew his sword to protect Jesus (John 18:10). Now, he has the option of joining John in witnessing what is happening to Jesus, but he can't. He's too afraid.