Albert Barnes Commentary Matthew 26:69

"Now Peter was sitting without in the court: and a maid came unto him, saying, Thou also wast with Jesus the Galilaean." — Matthew 26:69 (ASV)

Now Peter sat without in the palace. Mark says, the first denial took place while Peter was "beneath in the palace." This palace was the large hall or court belonging to the residence of the high priest. The part of it where Jesus and the council were was elevated, probably, above the rest, for a tribunal. Peter was beneath, or in the lower part of the hall, with the servants, at the fire. Yet, as Matthew says, he sat without in the palace—i.e., out of the palace where they were trying Jesus—namely, in the lower part of the hall with the servants—both narratives are therefore consistent.

And a damsel came to him. John 18:17 says that this servant girl was one who kept the door.

Thou also wast with Jesus of Galilee. Probably she suspected him from his association with John. This was in the early part of the trial of Jesus.