I have to question this on the following terms;
It is clearly implied that deployment occurs
before the game starts. Unless we chose not to read the "FIGHTING A WARHAMMER BATTLE" section [BRB pp204-.] chronologically, However I've never seen terrain placed AFTER deployment!
Well, if we do choose to read the "FIGHTING A WARHAMMER BATTLE" section (BRB p. 140 ff.), we discover that:
BRB p. 142: "Before you deploy any of your Wizards, remember to generate their spells," or as BRB p. 28 specifies: "Each Wizard knows a number of spells equal to his level, chosen randomly at the start of each battle."
BRB p. 149: "Reserves are not deployed at the start of the battle. Instead..."
BRB p. 150: "Roll off to see who controls the watchtower at the start of the battle. The player that controls the tower may deploy..."
Elsewhere we find:
BRB p. 79: "Deployment special rules affect how a unit is set up at the start of the game."
BRB p. 79: "A unit with the Ambushers rule does not deploy at the start of the battle. Instead..."
BRB p. 389: "They enter at any point on the table edge of one of the three areas in which the player deployed his units at the start of the battle."
Even the earlier identification of terrain is said to be at the start of the battle:
BRB p. 117: "The effect of a piece of mysterious terrain is not decided at the start of the battle. Instead..."
BRB p. 126: "As with other terrain elements, players should identify at the start of the game all pieces of terrain for which they are going to make use of the buildings rules given here."