Thanks, PvK.
I stormed it before I saw your message. Only units I lost were 2 swamp guards and 3 shamen.
I was really unlucky with the shamen. The star child astral dueled me 3 times and won all three times. I guess the attacker has an advantage? Or a castle defender has an advantage? Maybe the shamen are at a disadvantage underwater? Or maybe I was just unlucky?
I absolutely love this game.
Now to take Ry'leh's Last poorly defended sea province and destroy them forever!!! (Well...for this game at least.) I think I will send some shamen over to farm their void and see what I find.