---Oh, how beautifully it all sounds... go inside and crush the enemy... :)
But to crush the enemy behind the gates, you need to have more of your own troops there... but when playing against a human, Hydras will just stand at the broken gates (even better if they're not in the moat :)), slowed down by the slowing effect, and blocked by a pack of weak units.
Without proper MAGIC, ARCHERS, and SERIOUS "air" SUPPORT, capturing castles defended by a human will be extremely difficult... but the computer does it easily :)