rtfm!
1. I start a game, let's say I choose Rampart.
The opponent chooses Dungeon. Then, since I chose Castle, according to the rules, the opponent takes the remaining color, or vice versa. Then, the blue player starts choosing computer-controlled factions.
He takes Necropolis, Elemental, and Swamp. The other player chooses the remaining factions.
So, the blue player knows that one of the sides will have Necropolis, since he chose it. Therefore, the computer controlling Necropolis has an 80% chance of building a Black Magic Amplifier. Consequently, it makes sense for him to recruit a Necromancer in his tavern and level him up. That's what I meant.