I ran an expanded version of "The Haunted Keep" for my brother, a few years after we started playing D&D. A while back, I came across my decades-old notes, and scanned them, (downloadable, here).
Rough sketch of a plan for the West Tower of the Haunted Keep, drawn by myself, almost 40 years ago.
While nothing profound, the notes are of interest as a gaming artifact, and can serve as the basis for a more sophisticated revision.
There are a couple of good design elements (the hall of alcoves in the catacombs on the 2nd level, the caverns connected by an underwater passage on the 3rd level).
The stocking is quite random, with only some regard as to how nearby creatures interact, and a fairly rudimentary sense of dungeon ecology.
The surface ruins could be further detailed, with a rough map of the fallen castle walls, and a few hidden encounters (giant centipedes, a gecko, etc.)
This location could also serve as the site for an ambush, should the player characters leave and return to the dungeon, a few days later.
The West Tower:
Moldvay suggests using bandits on the 1st level, and both my original version and the Dragonsfoot adventure place them in the West Tower.
A well sketched-out group of bandits as one of the factions in "The Haunted Keep" is important, and since bandits may have an NPC leader of any class, this represents an opportunity to introduce a higher-level fighter, magic-user, or cleric into the mix.
Two of the prisoners should be held by the bandits, who confine themselves to the West Tower (and could be persuaded to switch sides).
The catacombs should predate the construction of the castle, as in Lovecraft's "The Rats in the Walls" (one of the methods to reach the lower caverns should involve a secret passage beneath an altar).
Perhaps there are dungeon cells, used by the Rodemus family for sinister purposes in the past, and where some of the prisoners are being kept, guarded by hobgoblins.
The various undead (skeletons, zombies, and ghouls) probably inhabit parts of the catacombs avoided by the wererats and their goblinoid allies.
The Wererat Lair:
These caverns should include areas predating the arrival of the Rodemus family, with evil creatures that even they and the goblinoids shun.
The Rodemus family should be sketched out as NPCs, with competing interests. Perhaps there is a power struggle within the family.
The hobgoblin king should be present, with thouls among his bodyguards. He has been overseeing the raids, and is mustering his forces.
The final prisoners should be here, although might be infected with lycanthropy (why else would they have been taken prisoner?)
Wererats, from the AD&D 1e Monster Manual. Illustration by Dave Trampier.
Each of the twelve prisoners should be fully sketched out (it can be randomly determined which of them are a specific player character's relatives).
Moldvay suggests "adding spooky noises and some ghostly figures which appear suddenly in odd places (though harmless)."
I've discovered that the best time for players to be told their characters hear a spooky noise (a low moan, a brief shriek, mindless gibbering, etc.) is when they are listening at a door (the noise originating in the distance from somewhere behind them, not behind the door).
The use of ghostly figures (deceased members of the Rodemus family, recognizable from old portraits, etc.) can help to convey that a curse has befallen the family.
Other adventures involving rats and/or wererats can serve as a source of inspiration (and ideas).
Dungeon #14 (November/December, 1988) "The Wererats of Relfren" contains a description of Greater Wererats (BECMI).*
*perhaps the bandits were delivering a shipment of "Emerald Moon Cheese" (green-colored cheese with holes in it)...
Knockspell #4 is devoted to rats, with adventures including "Beneath the Crossroads" by Joshua Gervais, and "Rats in the Walls" (AS&SH) by Jeffrey Talanian (inspired by Lovecraft's story, but not based upon it).