[Verjigorm]

Oh.... I went completely backwards on strategy then. That might be an expensive siege. I might think about running around on his lands instead of continuing to siege the castle with an army I couldn't defeat inside. You can't harass him to get rid of gems since the lab is inside. If it's an "extra" force, I'd keep the siege on to keep him inside the castle, but otherwise choose an alternate action because he can just keep summoning inside the castle.

The best strategy with a well-garrisoned castle is to try to draw his army out of the castle by attacking something else.

Sorry, forgot you don't have astral... If his dominion is weak, (after you have sacked his outlying temples), you can blood sacrifice until his dominion is gone rather than face his umbral army. Kill by dominion might be cheaper if you can manage it.

You're looking for the most economic solution. Take all of the easy stuff and leave him with his precious castle. Then surround his capitol with Temples and blood sac him out of existence. You can preach on his castle while besieging it to prevent dominion spread, but you might need a lot of priests if he has pretender and a temple or more inside.