The defender in the standard scenario "Surprise Attack" gets to move models onto the board in his second and subsequent turns. Here's the rule in full:
The defender, at the start of his second and
subsequent turns, may roll a D6 for each of his
Henchmen groups or Heroes not yet on the table. On
a 4+ they move on in the movement phase from a
random table edge as shown below – roll a D6. All
reinforcements for that turn arrive from the same
edge and may charge on the turn in which they arrive.
But here's the tricky part: Do these models set up within 8" of the board edge as per standard deployment, or do they set up ON the board edge? In other words, can they come onto the field 8" or 16" on their first turn? This is key for the Attacker player, as he needs to know whether his models 9" from an board edge are safe from sudden Defender appearance.