Peter went up ... - The small room in the second story, or on the roof of the house, was the usual place for retirement and prayer. See the notes on Matthew 6:6; Matthew 9:2. Even when there was no room constructed on the roof, the roof was a common resort for retirement and prayer. Around the edge a battlement or parapet was commonly made, within which a person could be quite retired from public view. "At Jaffa, the ancient Joppa," says Prof. Hackett (lllustrations of Scripture, p. 81), "where Peter was residing at the time of his vision on the house-top, I observed houses furnished with a wall around the roof, within which a person could sit or kneel without any exposure to the view of others, whether on the adjacent houses or in the streets. At Jerusalem I entered the house of a Jew early one morning, and found a member of the family, sitting secluded and alone on one of the lower roofs, engaged in reading the Scriptures and offering his prayers."

Dr. Thomson (Land and the Book, vol. i. p. 52) says of these roofs, "When surrounded with battlements, and shaded by vines trained over them, they afford a very agreeable retreat, even at the sixth hour of the day - the time when Peter was favored with that singular vision, by which the kingdom of heaven was thrown open to the Gentile world."

About the sixth hour - About twelve o'clock (at noon). The Jews had two stated seasons of prayer, morning and evening. But it is evident that the more pious of the Jews frequently added a third season of devotion, probably at noon. Thus, David says Psalm 55:17, "Evening and morning, and at noon, will I pray, and cry aloud." Thus, Daniel "kneeled upon his knees three times a day and prayed," Daniel 6:10, Daniel 6:13. It was also customary in the early Christian church to offer prayer at the third, sixth, and ninth hours (Clem. Alex. as quoted by Doddridge). Christians will, however, have not merely stated seasons for prayer, but they will seize upon moments of leisure, and when their feelings strongly incline them to it, to pray.

Acts 10:9 Parallel Commentaries

'God is no Respecter of Persons'
'And Cornelius said, Four days ago I was fasting until this hour; and at the ninth hour I prayed in my house, and, behold, a man stood before me in bright clothing, 31. And said, Cornelius, thy prayer is heard, and thine alms are had in remembrance in the sight of God. 32. Send therefore to Joppa, and call hither Simon, whose surname is Peter; he is lodged in the house of one Simon a tanner by the sea-side: who, when he cometh, shall speak unto thee. 83. Immediately therefore I sent to thee; and …
Alexander Maclaren—Expositions of Holy Scripture: The Acts

Whether Judiciary Power is to be Specially Attributed to Christ?
Objection 1: It would seem that judiciary power is not to be specially attributed to Christ. For judgment of others seems to belong to their lord; hence it is written (Rom. 14:4): "Who art thou that judgest another man's servant?" But, it belongs to the entire Trinity to be Lord over creatures. Therefore judiciary power ought not to be attributed specially to Christ. Objection 2: Further, it is written (Dan. 7:9): "The Ancient of days sat"; and further on (Dan. 7:10), "the judgment sat, and the books …
Saint Thomas Aquinas—Summa Theologica

Whether all Men Will be Present at the Judgment?
Objection 1: It would seem that men will not all be present at the judgment. For it is written (Mat. 19:28): "You . . . shall sit on twelve seats, judging the twelve tribes of Israel." But all men do not belong to those twelve tribes. Therefore it would seem that men will not all be present at the judgment. Objection 2: Further, the same apparently is to be gathered from Ps. 1:5, "The wicked shall not rise again in judgment." Objection 3: Further, a man is brought to judgment that his merits may …
Saint Thomas Aquinas—Summa Theologica