Which castle is the most difficult?

there are no "good" heroes

Are you just trolling? Tazar is one of the best heroes in the game!

a moat twice as wide;

But who really needs that moat twice as wide.

weak archers

Try playing as Vistan. And see how "weak" they are. Do you think the archers of the Castle are much better? So, play as Vistan.

But in this city, the focus is on defense.

High defense - yes. But the fact that you play defensively only speaks to your play style.

the hydra greatly slows down movement.

The hero should spend the night on a speed-boosting tile.

it is eliminated at a distance through mass slowing and blinding.

You can also use this against your opponent.