Use Phantom Dragon to destroy the gate for alternate win condition: Destroying the third gate equals breaching the stronghold and fulfilling the Invader's Victory condition.
As a necromancer you will be able to raise the most powerful of your units, The Phantom Dragon. He will be beating down on Stronghold's newfound gates, making it another possible route to victory. It will not stop. Each time you will put mana into summoning it, it will grow stronger. However, that's not the only way Phantom Dragon grows. He will be also powered up by panic on the walls. For ever point on panic marker this creature will grow stronger.
This expansion is a redesigned version of the old rules for Gate from the first edition of the game. It includes new rules for Invader and Defender side.
So brace for impact poor souls inside a stronghold, because the real nightmare just started.
Note: Although it was delivered in a small separate box, one of its components (a big gate token, which would not fit in this small box) was included INSIDE the KS edition of the base game.