1) Don't build KIRINS! I built them, but it was pointless! There's simply not enough money to buy them back... trust me, the rest of your army is more than enough to win!
2) Don't capture the upper-left castle! Let it remain as it is. The thing is, if you capture it, the Necropolis main castle will regularly send a ship that goes through the whirlpool and attacks the lowest castle! Otherwise, it will split your armies and attack the nearest castle on the mainland. I repelled 7-8 attacks this way.
3) Find Ash's Tear immediately! It will give you a transitional artifact that summons PHOENIXES!
4) Let the enraged orc capture your castle! HE BUILT EVERYTHING IN IT FOR ME!!!! The city was level 1 - when I repelled him, everything was built!!!