Posts from Tactical techniques for conducting battles in Heroes of Might and Magic II. Auto-translated

If the castle isn't too strong, trolls are quite helpful.
If it's stronger, then titans.

To capture castles without archers and flying units, there are three good options: the ballistics skill, an artifact that allows a catapult to fire twice, and the combat teleport spell. Using this spell on hydras usually yields interesting results:)))

In battle, you can also use this tactic: throw cheap units forward as bait. Preferably, use very durable units, like zombies, dwarves, etc. While the AI is busy dealing with them, it won't try to escape, and you can attack it with your stronger units.