Asklemmy

As many potential rooms as possible in as many locations as possible, and no way to distinguish the right one. Of course, rotate it regularly.

It would have to be well hidden or well guarded. Indiana Jones and Tomb Raider come to mind as well.

Some old palaces in Japan have "nightingale" floors that purposely squeak to keep people from sneaking in. Could be a security feature outside your room

Magic can McGuffin any answer you need but it sounds like you want a non magic means to keep people away.

Have a guarded door, be it creature or armed guards. The room that is guarded is not the real room, just a place to hold private meetings. It's used on the regular to keep the focus on the guarded room.

The real room is in the town's bizarre. It's passed by all of the time and is curated by the most hated man. No one likes going to his shop. He's rude and his wares are over priced. Within his shop is the real room that is visited by the few who know.

Is it on display in the bottom drawer of a locked filing cabinet stuck in a disused lavatory with a sign on the door saying, "Beware of the leopard"?

More seriously, just make it an incredibly uninteresting door that no one even thinks about, which everyone thinks is like for plumbing or linens or something boring and irrelevant. Then paint the door in whatever Disney NoSeeUm color is most appropriate.

Simply put: it stinks, like bad. The room is hidden under the largest city in the land, only accessible by spelunking through vast distances of underground sewers that smell like decomposing waste. This smell-field ensures only the bravest and most dedicated adventurers ever travel to the room, and none who attempt it enjoy talking about it.

Look at ancient Egypt. Big piramides didn't work. Getting your name erased from the society (Tutankhamun) somewhat.

The room is built on the inside of a giant snail's shell, if you give it a special kind of lettuce the snail will retreat and clear access to the room.

You can use the low-tech mythos ideology or superstition to keep people out

"We dont go in there because our forebearers spoke of misery to those that enter the room"

Boobytraps. Rumours about a curse/ghosts/entry fee. Pretend there's something boring in there. Allow entry, but make the process the full force of German bureaucracy (you need to file form A on a blood moon, to get form A, you need document B2, no not that one the green one, you can get it from Location C but that's only open once a month for half an hour, and if you don't wear pink they won't help you).

Guards with dogs. Cheaper than most alternatives, covers more ground, harder to confound, works against invisibility and disguises. With an advanced enough security protocol (against flying, burrowing, teleporting and magical disguises), it's nearly unbeatable.

Unless, of course, you don't want it to be unbeatable.

Put the room in an isolated location? In a cave, on a mountain, hundreds of leagues from the nearest settlement.

The whole thing is underwater

Not build a door or windows.

Depends on the context, like what world it is and where the room is located, and so on.

I suppose it's in a building that people have access to except for that one room, and that it's a world where, while not particularly sophisticated, explosives exist, so it should at least have very strong walls. If magic exists you might want to avoid using it for this purpose as it often becomes a "because" kind of explaination, but you can use it as a tool (e.g. an obstacle that can only be overcome with powerful magic, and the magicians who are capable of such power can be counted on one hand).

If magic is a no-go, maybe it simply requires a specific kind of key to open (but maybe it's too high tech for your world?)

• Strong locks on doors and windows.
• Reinforce doors and windows with bars or metal.
• Create hidden entrances or passages.
• Use guard dogs to deter intruders.
• Keep heavy furniture near entrances as barricades.
• Set up alarms or noise-making devices.
• Assign a guard or sentry to watch over the room.
• Use traps like tripwires or pitfalls.
• Conceal valuable items or use secret compartments.
• Explore spells or enchantments for added security.

Have you thought of a… lock?

Put a sign "staff only" on the door

Pit.

What does the room contain? If it's a valuable artifact, surround the room with rooms with mundane objects, and build the room around the valuable artifact without any doors or windows.

If the room contains some kind of social circle or government function (think the 5 elder stars in One Piece), I think you'd again try to make the room seem unremarkable, but this time post a guard INSIDE the door. Possibly with an antechamber between the actual room and the hallway so that it seems like there's just a guard in an empty room. Maybe you post many guards there and make it look like its their off duty room where they just hang out

Is it on display in the bottom drawer of a locked filing cabinet stuck in a disused lavatory with a sign on the door saying, "Beware of the leopard"?

You can spread powder on the floor so you could see if someone had stepped in. It wouldn't keep them out unless they didn't want anyone to know they'd been in there.

I think there's a more important question here... There are people who do access the room? Why? Who are they? That leads to the actually relevant question, how would those people secure a room?

If nobody needs to access the room, then the room shouldn't be accessible. By that I mean, it should be underground with no points of access, just an inaccessible underground chamber.