Having mentioned Witley Park and the mad things rich people used to make their landscape gardener and put-upon masons do, I was thinking more broadly about follies, the silly class of false features – pre-ruined pagan temples, artificial caves, monuments to nothing – that the wealthy enjoyed putting up around the place to add a little air of class and instant heritage. Here are d12 of them to use in your game, for populating the estate of the eccentric local landowner or the embarrassingly bourgeois guild merchant trying to look posh. Follies are good for making such landscapes more interesting and more gameable: the butler willing to sell his master's secrets can tell the party to meet him by the obelisk at midnight, or the ruined tower can provide a good place to stop and hold off the landowner's mastiff pack once it all goes south.
Sort of the point with follies is that they don't do anything, so mostly these should look half-plausibly arcane but not actually do anything. However, you might as well keep players on their toes and reward them for careful investigation: particularly if the estate has multiple follies, it's good to have one be secretly the real deal. So that stuff's in italics.
1 - A grotto. The grotto is the most upfront D&D of follies: a small fake cave (ideally tucked into a bank of earth behind a lake or something) complete with artfully carved stalactites around the cavemouth. Inside it goes for about 20ft of twisty, single-file passage, with a lightwell halfway down, and then just stops. Unless it doesn't. I'm sure you have a dungeon it can unexpectedly connect to if you lift the flagstone at the end of the passage.
2 - A neat little circular temple, with fancy columns at the entrance (one of them with the capital carefully knocked off for that historical look) and barely enough room for 6 people to stand inside: inside, opposite the doorway, is a vaguely ancient-looking block of carved stone with some candle stumps on it. The altar is fully operational and a hotline to Cyric (or whoever the local god of insanity is). If you keep an eye out you'll see the landowner's wife coming down here at midnight and lighting candles to stop the murderous voices in her head. She's only making it worse.
3 - Fake stone circle, with four trilithons standing (with the help of cement or mortar) and a fifth intentionally tumbled. A long, low stone in the centre has an altar-like feel, but the stonework on all of it is far too neat and fresh. Nevertheless, standing inside the circle and looking out through one of the trilithons shows the vista, and anybody peopling it, as viewed with True Seeing.
4 - A luxurious, tournament-style tent, the kind a king might take on a minor campaign, with rich blue fabric and a gold pattern stitched into it, the front half open and a curved bench inside. Except the canvas is actually plaster and the whole thing immovable. It does provide better shelter from rain than real canvas, though. Unfortunately it also has an unpleasant curse that attracts lightning during storms (or from spellcasting).
5 - An adorable little rustic cottage with a lopsided thatched roof hanging down and tiny, jewel-like windows looking out over the parkland: far too perfect to let an actual peasant live in it, of course, and mostly just a glorified garden shed full of tools (which make decent improvised weapons). Also the whole construction is about 2/3 normal human size: halflings love it here. Somewhere amid the rusting shears and rakes, findable with a bit of searching, is a wand of magic missiles, currently being used as the handle for a hoe: a cunning concealment until a local fence can sell it on.
6 - A modest, two-storey castle-style tower, with little battlements and arrow-slits and a flimsy arched door in the ground floor. Prettily decorated inside, for tea parties: the room's double height and the roof not accessible from the inside. If you can get to the roof, though, someone very paranoid has stashed 100 magical fire arrows and a longbow up there. It's a good spot for overseeing most of the estate.
7 - An adorably ruined tower, with a ground floor and then half of a neatly crumbled second one on top: built that way, of course. There's an archway on the ground floor but it's walled in and the tower is actually solid all the way through, with the arrow slits just cuts about a foot deep in the masonry. When moonlight hits the false door, however, it becomes very real. Somebody knows this, and they seem to be using the unreal chamber inside for storing an enormous amount of wealth.
8 - A whole mock-ruined castle: admittedly, a small one, with a couple of short towers and a curtain wall running between then and another spur of wall heading down a short slope before crumbling out. There's no interior, just the chain of neat-looking masonry on a small and equally artificial hill. But if you stand in the shell of the lefthand tower and make no sound at all, you can definitely hear the sounds of a savage and very real battle happening somewhere.
9 - A pre-dishevelled abbey, with the eerie “ruins” of a chapel of some kind, all high walls and long, empty windows, and around it much lower walls suggesting a wider floorplan and perfectly placed to take you out at the knees. But if you perform a service to Ilmater (or equivalent) at dawn you'll find yourselves with d20 temporary hitpoints each, lasting until sunset.
10 - A really foolishly extravagant fountain: mermaids, dolphins, a beardy man with a trident, the whole shebang, all horribly stained and ribboned with mould and weed. It completely dwarfs the fairly small round pond it's been placed in. The flowers of the water lilies that grow in the pond , when eaten, have a powerfully intoxicating effect, providing weird visions that create lethargy but boost Wisdom-based checks.
11 – A hermitage: a little rustic hut like 5, or possibly just an artificial cave with a door. Either way it's done up to look like an abode of scholars, with a silly conical roof or an inscription about wisdom over the door. It has an actual hermit too: that is, an eccentric old man with a long beard from the nearest village, whose adult children were only too happy to have him taken off their hands and given a meal a day by the local lord. He's pretty senile but knows what is expected of him and will do his best to talk in ominous ways. Except he actually is a wise and powerful sage, using the fake hermit thing as a cover. To hide from something? To observe the estate? To do some crucial research in peace?
12 – This place. Weird as hell. The local lord, or a predecessor, clearly has a mystical side. Silly little ceremonies where everything is in triplicate are held here every full moon, if the eccentric in question can find two people willing to play along. The construction is actually an extremely subtle and elaborate piece of architectural magic. If the party examines it carefully enough, they may realise that it triple-binds an enormously powerful demon by folding it into the angles of the triangles, storing it between dimensions. So why the ceremonies? Do the bonds need sustaining? Or is someone trying to free it?