Necromancer – “Invasion” [Campaign 3, Mission 3]

Playing for the first time, version 1.5. Heh! I got the Staff of the Underworld on the island; it was guarded by bone dragons, who happily joined my black parade. And how do I turn an Academy (I captured the first one in the west) into a Necropolis? Right now, I'm facing a considerable army of mages, and I don't have time to run away; I urgently need a Necropolis. Who can't repel an invasion on their home island? I let a mage capture a Haven, then he went for the Necropolis, leaving the garrison empty (why, I don't know). Isabel captures the Haven and waits for the mage, with his army tired from two sieges, to return. It's easy to repel the siege.