Are you sure about making multiple fortresses? It seems to me that the necromancer/archmage archetype (as opposed to, say, an emperor or warlord) prefers to build upward rather than spread out, creating a single ominous palace whose influence spreads out like a dark cloud of notoriety (or a literal dark cloud) that covers and corrupts the land around them. It suits their standard issue ego much better. Who needs tax collectors and local provinces when you have mastery over the fabric of reality itself? Villages who fear you should come and offer you tribute themselves in exchange for not killing them.
Perhaps distant battles could be abstracted (You send X zombies/monsters/slaves to make an attack on a village with Y defenses, roll for success, gain some fresh corpses and prisoners, lose some zombies, build summoning circles to build magic power, use magic to turn corpses into zombies or summon demons, and so on), while armies who attack your stronghold directly could play out in real-time like cross between a tower defense game and an action RPG (you could have zombies, monsters, archers, etc defend your tower automatically, while you stand at the top of the tower and cast long-range spells to fend off stronger enemies). It would also be cool to have some long-term notice about heroes or teams of heroes approaching before they actually reach you, and you could send out bands to try and intercept them before they reach the tower. Unlike standard 'defense' games, though, who attacks you would be based on how you treat the surrounding area, who you start wars with, etc., instead of just sending stronger and stronger waves at you constantly.
I'd say the ultimate driving force of the game, once you have built up a strong zombie army and have achieved high enough notoriety among the surrounding villages, should be attaining more and more power from more and more powerful demons. Perhaps you can choose to either cut deals with demons, or when you get strong enough, actually capture them and force them to do your bidding (which would be a very dangerous endeavor that could potentially backfire severely). Maybe an endgame goal could be building a tower to heaven and waging war on the gods of the setting.
Also, maybe you don't like this idea, but what about a 'benevolent archmage' option?