Regarding the castle development, I agree with Machete. When building an army, I prefer to recruit vampires and liches first. Zombies usually just accumulate quietly in the castle (by the time they crawl to the enemy in battle, it's usually already over), so I only buy them when I have a decent amount of money. Undead and dragons are, of course, a good thing, but you usually can't fight with them until the middle of the mission, when the necessary buildings have been erected and you have enough funds. I always besiege the castle, reinforcing the army of vampires and liches with some ghosts; their towers always shoot at them. I think everyone has a roughly similar tactic, and Machete has already described it in detail.