In Heroes, the balance is NOT IDEAL!!! (a prime example is Necropolis and Conflux). In most normal championships, these castles are banned.
To take castles with this city, take Ballistics... it's often given in this castle. Break through the gates in 1-2 turns, and crush all the enemy units that have gathered inside with Hydras!!! ---Ah, how beautifully it all sounds... climb inside and crush the enemy... :)
But to crush the enemy forces behind the gates, you need to have more of your own troops there... but when playing against a human, the Hydras will just stand at the broken gates (even better if not in the moat :)), slowed down by the slow effect, and blocked by some weak units.
Without proper MAGIC, ARCHERS, and SERIOUS "air" SUPPORT, it will be extremely difficult to capture castles defended by a human... but the computer does it itself :)

I must admit, I haven't played with Rampart for several years ;)
So, I just play sometimes... on S-levels against 7 computers... That's why, when discussing castles, I don't even talk about playing against the computer... I don't see the point in it...

Slowed down by the slow effect... And we'll rush them... or heal them, depending on what we get... we still have important early levels - don't say that we don't have magic at all... Tanya was right that water is often given here... And if you're not an idiot :D, you won't put a Hydra in the moat to stand there ))) good flying support is available... Vipers and Serpents....

And I disagree about the imperfect balance... There is always a solution for everything... For Necropolis, there is a tactic (discussed in particular in the topic "How to defeat Garthan"), for Conflux, there is the expert spell "Curse"... It's just that it's easier to play against some factions, and harder against others, but there are NO BAD CASTLES! THERE ARE JUST PEOPLE WHO DON'T KNOW HOW TO PLAY WITH THEM ;)

Added 2 minutes ago
And playing against Swamp on a Random HL... oh... How many Vipers do you think I can gather in the Serpent Nests????