Krdytkyu

I'm no engineer, but I was wondering how a steam-powered door would work. I want it to be difficult to open unless you know how (not a simple "twist the doorknob and open"). There is access to plenty of water and fire in this scenario as well.

Hero of Alexandria (c. 10 CE – c. 70 CE) was a Greek mathematician and engineer who was active in the 1st century CE in his native city of Alexandria, Roman Egypt.

He wrote several books describing very advanced machines, including a fully automated theater play performed by marionettes actuated "by a binary-like system of ropes, knots, and simple machines operated by a rotating cylindrical cogwheel" (Wikipedia).

More inline with the question, he described an engine which "used air from a closed chamber heated by an altar fire to displace water from a sealed vessel; the water was collected and its weight, pulling on a rope, opened temple doors". (Wikipedia)

The construction of the machine is described in his book, Pneumatica, section 37.

The construction of a small temple such that, on lighting a fire, the doors shall open spontaneously, and shut again when the fire is extinguished. Let the proposed temple stand on a pedestal, A B C D, on which lies a small altar, E D.

Through the altar insert a tube, F G, of which the mouth F is within the altar and the the mouth G is contained in a globe, H, reaching nearly to its centre: the tube must be soldered into the globe, in which a bent siphon, K L M, is placed. Let the hinges of the doors be extended downwards and turn freely on pivots in the base A B C D; and from the hinges let two chains, running into one, be attached, by means of a pulley, to a hollow vessel, N X, which is suspended; while other chains, wound upon the hinges in an opposite direction to the former, and running into one, are attached, by means of a pulley, to a leaden weight, on the descent of which the doors will be shut. Let the outer leg of the siphon K L M lead into the suspended vessel; and through a hole, P, which must be carefully closed afterwards, pour water into the globe enough to fill one haif of it.

*It will be found that, when the fire has grown hot, the air in the altar becoming heated expands into a larger space; and, passing through the tube F G into the globe, it will drive out the liquid contained there through the siphon K L M into the suspended vessel, which, descending with its weight, will tighten the chains and open the doors. Again, when the fire is extinguished, the rarefied air will escape through the pores in the side of the globe, and the bent siphon, (the extremity of which will be immersed in the water in the suspended vessel) will draw up the liquid in the vessel in order to fill up the void left by the particles removed.

When the vessel is lightened the weight suspended will preponderate and shut the doors.

Some in place of water use quicksilver, as it is heavier than water and is easily disunited by fire.

(Hero of Alexandria, Pneumatica, 37, translation by Bennet Woodcroft, London, 1851.)

Left, a 17th century German portrait of Hero of Alexandria. Right, a diagram illustrating Hero's pneumatic temple doors, as depicted in the 1851 English translation by Bennet Woodcroft.

Well, if you want a puzzle or lock of some kind, then do that yourself, but the basic action of opening a door is still "Push/Pull/Activate (Input Device).

Steam is pretty good at pressurizing, but as with all gases, it will want to expand before pressure starts to rise above normal. If you make it a bit longer than it needs to be to fill the doorway, you can fit a hollow space in the bottom. Place that over a chamber with valves to your boiler. When the steam is released into the chamber, it will push up the door with an action that might look similar to a trombone slide. When it's at the top, it can be locked and held in place. Lowering can be as simple as pulling a lever to release the lock.