Round table

In my opinion, Rampart, Castle, Dungeon, and Swamp have very good initial offensive power. Each of these castles has its own starting, powerful units, with which you can gain a good momentum...
I can't say anything about Necropolis and Inferno, as I don't play with them; they have weak castles, and accumulating skeletons for offensive power is just a waste of time...
It's very good to start with Tower, as it has a powerful unit right away, and if you also find a second native hero in the tavern, then you can gain a significant momentum...
You can play with Stronghold under certain conditions, which often don't work out for me, and like with Inferno and Necropolis, you need to know how to play with these castles!