"Doctor, it hurts when my players do this..."
"Tell your players not to do that anymore."
KotB was designed for a VERY different style of play than is typical these days, especially with other editions. Lots of good suggestions have been given - make adjustments to the monsters, henchmen and hirelings, provide clerics at the keep regardless of whether they were there originally, sell the PC's a few potions regardless of whether they were available at the keep originally, etc. HOWEVER, the point is that THE PLAYERS are learning absolutely nothing. THEY are not looking for alternatives or answers when they absolutely should be.
First thing they damn well should have learned by now is that just blundering in repeatedly, like bulls in a china shop and trying to smack down EVERYTHING they come across, is a lethally dangerous, and painfully protracted process. THEY AREN'T UP TO IT. If they insist that is the ONLY way for them, then they also accept the consequences of that refusal to improvise, adapt, or overcome the very obvious obstacle they insist on beating themselves to death against. That's where henchmen, hirelings, and ALTERNATIVE COMBAT strategies come in. Rather than just barge in with a battlecry they should lure the enemy OUT to where the PC's have ALL the advantages - like to doorways or cave entrances where the monsters have to bunch up and the PC's can throw lots of missiles at them, or flaming oil, or rig up traps, or whatever. ANYTHING but just the same things they've been doing that they KNOW aren't effective.
Second, stop fighting PERIOD. Try sneaking instead. STEAL the loot and AVOID the fights! Scare the monsters away instead of insisting that nobody gets out alive. BRIBE one monster group to fight FOR the PC's against another monster group, and agree to then leave the area. People might pooh-pooh diplomacy - but have they actually TRIED? It's one thing to try and fail, but another to try and fail and then never ever try it again, and still another to NOT try and then insist it'll never work or you're no good at it.
If one entrance is just impossible because it's too dangerous - LET THEM DIG A NEW ENTRANCE. Use those dwarf and gnome mining skills.
Really, these are not problems with the adventure nearly as much as they are SEVERE problems with unimaginative, and un-interactive players. It should not be the job of the DM to THROW solutions and alternatives at the players - the players should be ACTIVELY seeking out alternatives without even being prompted to do so (except by their own obvious failures of reasonable success. However, as the DM in a situation like this, perhaps the best thing to do is throw in a friendly NPC observer - a tavern patron who has now WATCHED the PC's come back to the keep repeatedly with their butts dragging. They can then offer some friendly advice, "Ya know, there's a lot of swords for hire here in the keep - even in this very room. They might fight pretty cheap and help you make som headway in your quest." "Have ya thought about going outside the keep to another nearby village and hiring a cleric to come with you, or just have them wait here in the keep to speed up your recovery?" "I was thinkin' ya might try the SNEAKY approach. Once you steal their best loot, a group of monsters like those might decide there's better places to do their raidin'."
It's all still the players own choices what to do, but if they keep being STUPID about their approach to the adventure, then you can at least righteously gripe back at them that you've given them all these different ideas and the only one they choose is the one they KNOW doesn't work.
