I want to run (probably online) an OSR game set in a castle under siege, and the dungeons below. Maybe I'd use OSE, or maybe I'd find a system more suited to doing this exactly. Maybe I'd hack something together based on the GLOG and the rules system for On A Red World Alone.
Basically, the castle is under siege. The castle has a vast and unexplored dungeon underneath it, probably mostly bricked off. As a sort of hail-mary to try to withstand the siege, the castle authorizes some of its people to go into the dungeons and try to find anything to help the castle.
I'd have to make a few adjustments. For one, being in a castle under siege means supplies are limited. Every ration eaten, every sword broken in the dungeon, every arrowhead blunted when it hits stone, is gone forever. Early in the siege, this might not matter; the castle is well-provisioned. Later on, it will matter a great deal.
Similarly, there's no use for great hoards of gold and treasure. Piling up more gold in a besieged castle only serves to reward the attacker more when they take it, unless you can use the rumors that you have it to convince mercenaries to break the siege for you.
Probably, I'd want the progress of the siege to be an ongoing thing that affects the campaign overall that's adjusted session to session. There'd be things players could find that would improve the situation in the castle, of course, but those would be major accomplishments over several sessions.
I might want to use a system which emphasizes the abilities of individual characters over using large numbers of henchmen. The castle doesn't have an endless supply of personnel to send into the dungeon, and, as mentioned, gold isn't going to be persuasive in terms of getting them to send more people down. Still, I don't think I want PCs to be entirely alone; each PC having a small group of henchmen, of which they bring one at a time, so that they can level up their successors should they die, is probably ideal for me.
There might need to be some rules reconfigurations. I might need a different unit for the EXP value of treasure than monetary value.
I'd probably also want mechanics for a shared spell library. Rather than individual magic-users having their own known spells, the castle knows a bunch of spells (and by retrieving spells from the depths, you can add to this) and individual magic-users can roll to understand spells from the castle's library.
I'm envisioning things like how in Ultima Underworld 2, the castle (encased in a blackrock dome and completely impassable in or out) starts getting water provisions from the ice caves you find a portal to, or perhaps you find an underground magical garden where you can supplement the castle's food supplies with crops that surely won't have any horrible side effects when surfacers make them a large portion of their diet, a passage to the underdark where you can parley with Drow, or even just an alternate exit to the dungeon half a mile to the north, which the attacking army has neglected to encircle.
The campaign ends when the siege is broken, through whatever means, when the castle falls, or, perhaps, by other means, such as evacuation through the dungeon.
