I think if I were to run it again, I'd devise a timeline with a countdown to the attack on the keep, led by the evil clerics and their humanoid army. And run it for 3rd-5th level characters.
I did precisely that, the last time I ran it (for my still running, 11 year old PBemail campaign). That fight was the best thing in the campaign -- took about 220 rounds, all told.
The party was only about 2nd level, for the most part, during that fight, but I kept them alive with a plentiful supply of healing potions -- and the players did their part to use the Keep's defenses, doing things like tossing alchemist fire from behind battlements at the approaching zombie horde!
The fight ended in the Great Hall of the main building of the keep, with undead coming in the bashed in doors, and players and NPC's preparing for their stand . . . but the enemy ran out of undead before the PC's ran out of potions. The ending was like the ending of "Platoon", desolation everywhere, but they somehow mostly survived!
BTW, not that it matters, but my version of the Keep was in the World of Greyhawk, an outpost of Bissel in the lower reaches of the Yatil Mountains. This was towards the beginning of the war between Bissel and Ket in my variant version of the Greyhawk Wars. The clerics were of the Baklunish deities, which in my campaign are Greek -- Hecate, the goddess of evil magic and Ares, the chaotic evil god of war and mayhem. The PC's have puzzled ever after as to whether the bandits, various tribes, and necromancer in the Caves of Chaos were supporting Iuz, Ket, or some other faction . . .
In the end the PC's made a peace deal with the king of the Goblins, which has lead to peace in the area . . . and gives me an unorthodox way to bring in the Red Hand module if I ever want to, with a warning from the goblin king.