The strongest castle in Heroes of Might and Magic IV.

In my opinion, all castles are good. And each one can be played against any other.
The 2 lines of castle development were not given by chance.
Tip: play random castles more often, and you will like them all.

Black Dragons are very good creatures, but the growth is 1 per week and they are expensive (8000 gold). Therefore, they can be killed by sheer numbers. (Usually level 4 creatures have a growth of 2 or more, while Black Dragons have 1)

OptiMAL is correct. Black Dragons are easily killed by Phoenixes.
1) There are more of them.
2) Fire immunity (50% resist to fire attacks, and Blacks attack with fire.)
3) Resurrection. (poor, but at least some)
4) Buffs for Phoenixes; for Blacks, there is only tactics.