If you think it's tough in the sewers, wait until you see what awaits you at the castle as soon as you escape.
The first battle at the gate is outrageous (especially since waves continue to arrive until you blow up the gate), but what comes after is simply beyond the pale. I may get bored enough to check the EL of the encounter like I did last time, but I remember that it said when it rolled everyone's initiative that it is an EL 24. No way. Even if it is, we're only like level 17 by that point. 7 levels above the party and the enemies are fully prepared for us with self-buffs.
Imma admit right here that I cheated and I STILL couldn't make it through all the waves of enemies. I could have cheated more and harder, but all of my guys had every feat available by that point, stats approaching mid-30's, stacks of arrows of slay everything with +4 bows of wounding and speed, amulets of blindsight, and they still tore me a new one.
At this point it's almost certainly devolving into a cheating simulator, not a RPG. I dare you to show me the party that can make it through this game without cheating in any way.