When they were past the first and the second ward, they came unto the iron gate that leadeth unto the city; which opened to them of his own accord: and they went out, and passed on through one street; and forthwith the angel departed from him. The first and - second ward - It is supposed that ancient Jerusalem was surrounded by three walls: if so, then passing through the gates of these three walls successively is possibly what is meant by the expression in the text. The prison in which he was confined might have been that which was at the outer wall.
Iron gate - This was in the innermost wall of the three, and was strongly plated over with iron, for the greater security. In the east, the gates are often thus secured to the present day. Pitts says so of the gates of Algiers; and Pocock, of some near Antioch. Perhaps this is all that is meant by the iron gate. One of the quaternions of soldiers was placed at each gate.
Which opened - of his own accord - Influenced by the unseen power of the angel.
The angel departed from him - Having brought him into a place in which he no longer needed his assistance. What is proper to God he always does: what is proper to man he requires him to perform.