We were actually writing at the same time:)
As for the castle, I don't know. It usually helped me; you can also arrange a kind of relay race with several heroes and transfer troops to the main city in 1-2 turns...
Well, if you capture it early, it's not necessarily at the expense of your own. I understand what you mean (you have little money because there's only one capitol, and there might not be enough mines), but it's still always better to have two castles than one. Usually, I focus on my own, and then I build up the captured one; sometimes, however, I develop them almost in parallel, and after building the Necropolis, I try to quickly build up to level 5 in the new city:)