A Door a Dozen

Secret doors are a hallmark of the dungeoneering experience. How much more fun is finding a treasure hoard than to have found it after having found it in a secret place? How much more rewarding, knowing others must surely have missed it before you didn't! And one of my favorite things with secret doors: coming up with fun, special mechanisms with which they open - and thematic yet sign-posted descriptions that may allow the player to find it with no roll being made.

That said - it's not always easy (or feasible) to come up with a thematic and unique secret door on the fly: so herein provided - a dozen secret doors (and the secret behind their opening!)

1d12	Secret Door
1	Table In the Floor In the center of a space is a table - supported by a single leg in the center, affixed to the floor where it stands. Optionally, something may be atop the table - a cloth, votive or otherwise, alternative folderol. The table will turn, however - if a character attempts it - and rotate: after three or four rolls, it becomes evident that the table is rising. The table is on a dais which - if rotated for at least 2 turns (1 turn, if two or more allies turn it together) - will rise, allowing egress into a hidden room below. Beneath the table dais is a chain leading down to assist with the climb.
2	Ferrous Curtain Rod Against the wall in which the secret door is found can be seen hanging a thick curtain - suspended from rings along a metal rod running the length of the room. The walls are paneled and, if the curtain is closed, behind the curtain - a secret door, one of the panels, is open. The mechanism to the secret panel, however, is attached to a magnet hidden inside the suspending rod. If the curtains are opened, the panel will shut - concealing the egress; if the curtains are closed, the panel will open as the magnet moves to the other side. A character may detect this hole by prodding the curtain wall externally, noting a dip when they prod the section with the opening; by going behind the curtain without opening it; or by noting an odd sound that can be heard when opening or closing the curtain.
3	Sliding Stonework Inlaid in the stonework of the wall is a false section of masonry, detectable by an observant hand, noting the mortar suddenly becomes smooth in that section, or by the temperature of the faux stones - which is slightly warmer than those around it. To open the door, one may push it in: but it will only go so far - not quite far enough to open. At that point, it can slide either left, right, or up. Left or right result in the door becoming stuck - up will allow it to click into a latch on the far side, securing passage through.
4	Under the Surface In the center of a space is a tiled pool - 20' long by 10' wide by 10' deep: the bottom is white - the sides are a checker-board: black and white, in 3' hexagons. On one of the long spaces, however, a black tile isn't a tile at all - but instead a hole where a swimmer might swim through: shadows of the walls, poor lighting in the dungeon, and the distortion of its depth conceal it from casual observation. The party might find it if using a "full daylight" effect, which would eliminate the shadows; but throwing a non-extinguishable light source down into the water, or by prodding around the pools edges with a lengthy pole or a spear. Swimming through results in a short tunnel, water-filled, which terminates in a new space: the waterline of the new egress equivalent to that of the pool on the other side.
5	Arch Trapdoor The arched ceiling of the room is tiled with particularly large stones for an arch. Detectable by chance or by drafts influencing the smoke from a torch, one of these tiles is false - significantly lighter than the others - and can be pushed up. If pushed all the way - it will fall, the arch being on a slope, after all - and a falling block, though it will not break, may attract a wandering monster. Once open, though, a man with something to stand on can climb through with relative ease.
6	Mouth of the Monster The sculpted head of a crocodile or similar monster, a headdress of some archaic make donned atop its stone head, sits, mouth closed, on one side of a chamber. Its eyes are closed and - if a character looks - they may notice four small indentations on the floor, where once a four-legged brazier stood. Ash stains or rust in the ruts may be present, included as hints for the party. If the party makes a material "sacrifice" - that is, by immolation, destroys an organic substance between the studs, the eyes of the statue open and the mouth slowly rises up, revealing a passage down the gullet. The trigger is not necessarily magical - could be heat-driven, based on the specific location and intensity of the flame. The mouth will remain open as long as the fire persists. Other potential hints to the presence of the secret door may include rails for the mouth to slide up on the wall or floor - as well as deep grooves in the statue, itself: deeper for the eyes and mouth than for the nostrils, etc. - to account for the joints concealed beneath.
7	Illusory Pocket Ringing the plaster walls of a room is a wooden runner. Beneath the runner, below where a character might lean on the wall, but above the trim such that it might not accidentally be kicked or prodded with a scabbard, is a small section where the wall is not. Instead, by lighting (or magic!), an illusion of wall - the color continues, the texture to the eye, but the physical wall does not: such that a character patting the wall may find the hole. Depending on preference, the hole may be large enough to accommodate a squeeze - or, perhaps it simply hides a lever which opens a larger door, hidden in the corners and seams of the room's wall, proper. Optionally, include a mechanism in the space or surrounding rooms that may serve to elucidate the portal.
8	Gate of Vines A wall of bars - or grated partition - is totally overcome with vines. Growing out from the mortar itself, they have totally taken over: forming a thick layer of green over the vertical space. In the ironwork is situated a hinged door - not even locked! - but the preponderance of vines has made the gate invisible. Potential mechanisms to detect the gate may be the elimination of the vines, wholesale (through Fireball or similar), or perhaps by noting a squeak while applying pressure in the vicinity of the gate (the vines will cause the gate to flex, its hinges causing the sound).
9	Put the Candle Back! In the wall near to a book case or other standing furniture is a candle holder - in which can be found a half-used candle. The candle is depressing a pin which - if the candle is removed, will retract, causing the book case to cycle like a revolving door, transposing anyone within 7 feet of its center to the other side of the wall. Another book case of similar contents and build is present on the reverse of the wall - and thus the door is once again hidden when the rotation is complete. The candle is outside the radius and is thus inaccessible from the other side of the secret door.
10	Molding Fountain A fountain is embedded in the wall - water dripping rather than flowing - out of a gargoyle's mouth into a stagnant well - hemispheric, 18 inches or so in diameter. Mildew and mold grows atop the sides and across the surface of the water: obscuring view into the space. If a character reaches into the water and feels around, they will find a lip on the rear side - directly under the mouth of the grotesque: inside this hidden alcove is a pull chain which mechanically withdraws a pin, allowing the character to push the fountain inward: revealing entrance to a hidden space. Optionally - the fountain should start working again (that is, water should flow from the gargoyle's mouth rather than drip) when the door is in an open state.
11	A Gap in the Fireplace Against one wall, a fireplace - wide mouthed without a lip or ledge - is embedded. Inside, where the flume lever might be, there is a second switch: one which, if turned, opens a panel to one side of the fireplace, the seams of which are obscured by soot, and allows egress into a hidden space. A player might find the space via a draft coming through the empty fireplace; by observing that embers, uncleaned, appear to have been blown in a singular direction, opposite to the draft; or by chance - thinking to open (or close) the flume and light a fire.
12	Shadowed Portal When in an illuminated state, the room and wall seem entirely normal. Optionally, the referee may include a suspicious stone arch, as though a door was once there, that has been bricked shut. However, in the absence of light - magical, mundane, or otherwise - the arch is open: unsealed; providing a way through. The party may find the secret door if a character with infravision is scouting ahead, if a Darkness spell is cast (by mistake or by malice of a wandering magician), or - if presented with the optional hint described above - by chance if the torch flickers, as if the characters are resting in the space, exchanging a dying torch for a fresh one, seeing the masonry blocking the doorway likewise seem to fade in places where shadows fall as the light is dying.

Public domain or open license artwork retrieved from OldBookIllustrations.com and Pixabay and adapted for thematic use. Attributions in alt text.