Getting Blood from the Stone

Not.
>If you're reading this, it's already too late

have there be more parts to the watchtower than the party knows about

Make it huge and under sedge.

Deep Space Nine, though you might have to make the watch tower sort of magical to fulfill the average plots of the DS9 which were basically:
>the crew is trying to get some bit of the station working again, this causes some long dormant magical effect/creature to pop out and cause havoc
>someone comes by the tower on the way to elsewhere, and either causes interpersonal drama among the crew or brings something that pops out and causes havoc.

To reference something that should probably not be referenced in terms of suggesting "good story setups", Black Clover has the MC join up and be member of a group known as the black bulls who are generally mercenaries, but they live in this old haunted castle whose rooms move about mysteriously and does weird shit at times.
So basically have the watch tower be like that: it was some old wizard's tower before being made a watch tower, from the outside it's only 3 floors high, and while you can get to the upper parapet where the signal fires are kept by just going up two short flights of stairs, there's a hatch at the top of the tower that opens up to another 10 or so floors that change and shift around at whim, and anyone manning the tower has a choice between trying to live in the cramped but vaguely normal area of the tower, or trying to inhabit the wacky ten floors.

(and they find an old journal early on in the wacky place like says, but there's also a map with "treasure room" marked and underlined many times on one of the more distant rooms - the map is of course unhelpful, as it begins with you going through a door on a wall that isn't there any more in a room that doesn't have the same number of walls as on the map but the doors are described, if vaguely in terms of material & colors etc.., and those do reoccur so the map can sort of be deciphered.)

Mix those up with normal duties and some PCs having to stand watch while others go into the side rooms, as well as visitors.

Make it really, really, really tall.

Windows show subtly different landscape than if you stand outside.
Each window changes it in different way.

I agree. The premise is grounded enough that the DM can easily improvise a narrative and open ended enough that the party can take the game where they want.

From the outside the tower appears to have several sets of floors, some of which appear inhabited, but the PC's only have to climb one flight of steps to reach the top. Looking down from above reveals that you have climbed to the top, despite not having gone up enough steps to justify the distance.

>The tower is the first border checkpoint in the road to the glorious kingdom of Arstotska

I recognize that lighthouse. Fucking chaurus.

They all start turning against each other over time. What looks like cabin fever turns out to be the tendrils of an old God driving them them to madness. Have it go full The Shining, meets Frostflow Lighthouse.

How about a diary that lists the actual daily lives of the party told through an unseen third party. Only, as it goes on there are slight changes and inaccuracies. The further it goes the more bizzare and violent these inaccuracies get, eventually ending in a bloodbath. Surly this won't start happening, right?

It's nearly the ten year anniversary of The Wall thread.

suptg.thisisnotatrueending.com/archive/1034444/

Pretty easy setting to do a sort of cabin fever story. Say there's like 4-5 guys, and these guys are cut off from the rest of the world. They start going a bit stir crazy, maybe they get paranoid or spooked, one guy is injured in a relatively mundane fashion but it gets infected or something. It would be a bait & switch, where there are implications of supernatural horror but that's actually just the onset of natural paranoia and social seclusion.

Have tense scenes where delirious injured guy hallucinates monsters, scenes where a guard stands on the edge of the tower as if contemplating suicide, scenes where the guards appear to be being watched by some third party.

You make it comfy.

>The Watchtower gets regular visitors from across the Empire.
>Merchants, Caravans, and lonely travelers all stop to pay their respects, and their taxes.
>They regularly meet wild characters, who share news and stories of their travels.
>You play as these travelers in one shot sessions as you reenact their stories, not often without some mighty embellishments.

>The men who maintain the watch must also maintain the tower.
>They must gather food from the forest, hunt deer in the nearby clearings, and travel to the nearby waterfall to gather fresh water.
>Sometimes queer things appear in the wilderness, not all of them friendly.
>But some of them are.

>Boredom is the greatest threat to a Watchman's health.
>The Watchtower has become an infamous place to after you're superior officer beat the famous pirate Glint Stickypalms to a game of cards.
>Now rivals and competitors all come to challenge the famous Watchmen to test their luck.

>The town in the distance is quite the treck, but is often made for supplies.
>The occasional bandit lies in wait for those that pass by.
>But mostly you'll meet the local townfolk. Hardy and hospitable.
>Haggle with the merchant, flirt with the baker's daughter, or do odd jobs for booze money. There is always much to do in civilization, but don't linger too long,
>Spend to much money, forget a vital supply(including the Captain's tobacco), or head out to late before sunset, and the Captain will have your head.

It's the last watchtower.

Civilization has fallen (you're not sure to what. The messages you received were confusing.)

Now you've got a little food, a little time... and now the horizon's burning. And it's slowly coming your way.

This might actually make for a pretty interesting game, but you'd need the right players to run it with.

Fuck, I'm old.

Make it seem normal at first.

Then create coincidence after coincidence. Mystery after mystery that somehow involves this little watchtower.
Is there something special about this tower?
Maybe.
There isn't much proof about it - maybe some old notes from people who had done watch before, complaining about "this fucking tower"
Did they know it was built over an ancient dungeon full of creatures of the night.
Did they mean to construct it out of mystical stone cut from an ancient rock formation used for worship, still retaining some ancient runes?
Surely they didn't know that it would watch over the only road out of the valley if the century old dams miles away broke and flooded the lands.
It's probably mere coincidence that the two warring armies of nearby armies will be converging after one discovered that this area is a great way to storm their enemy by surprise.
Or maybe they did.
But it won't matter for those guarding this place and staying watch.
For them it will be hell.

What sort of system would be best for this? OSR? AD&D? 5e? I'm actually really interested now in a seemingly generic fantasy setting or something a bit more darker like Warhammer Fantasy's realm with this plot and trying to build a campaign myself

Isn't that essentially what Ten Candles is?

I love the idea of a fantasy deep space 9 set on a massive tower on an island.

Warhammer Fantasy 2nd Ed. would be perfect for it, imo. Could easily set it in the Warhammer world, but not necessarily.

One day the mundane routine leads to not so mundane discoveries