Well, I don't like Castle but I have to admit that it is the strongest of all city types.
I will go through the troops again...
Halberdiers are second best 1st level units. Some can say that pixies or gremlins are but actually those are good only because of their special abilities and halberdiers are good by themselves.
Archers/Marksmen... Well, those are nice shooting units. I'd even say best 2nd level units.
Then there are griffins. Since they are Castle featured troops, they can be raised in large quantities and thus make another good unit.
Crusaders are very strong level 4 unit. 2 attacks make them a real threat especially if they strike after another group. Still they are slow and this is their disadvantage.
Zealots are not very good troops. They are too costly and their quantity is too low to make them a considerable threat.
As for Champions - those are superb. Their blazing speed and range bonus make them fearsome opponents.
I won't say much about Archangels - their stats and resurrection ability makes them strongest 7th level unit in the game.
As for Heroes - I rarely play Castle, only in campaign games, so I don't know much. People say that Sir Mullich and Lord Haart are the best. I play with Lord H usually and sometimes with Kuthbert...