Realism, in an RPG, is mostly a mirage. Even if the dice could do more than broadly approximate probabilities in the real-world, most players – and GMs – would not want to subject their characters, for example, to the actual risks of death and injury in a medieval battle.
Some groups aim for genre emulation – heroic fantasy, anime, pulp, horror, and so on – as a touchstone. But genre is a literary and narrative construct, and so it can lead you to try and meet the expectations of the story rather than find the story by playing the world.
In the fictional worlds of the games I play, I aim for _verisimilitude_, the appearance or sense of being real, which comes through consistent and believable details that emerge from real-world experience and expectations. Verisimilitude creates the impression that things are plausible without trying to accurately simulate the precise parameters and probabilities of any given event, like a sword-swing or a hike on an icy mountain path.
So in pursuit of verisilimilitude, it's a good thing to check your expectations against the real world. Over the years, I've gone into dungeons (like the catacombs of Paris), castles, towers, and recently the bunker forts of the Atlantic Wall, and each time I'm reminded of some useful, and grounding, facts that we can apply to our fantasy adventuring sites.
Passages, chambers
Passages in fortifications tend to be narrow – and cold. In your dunqeon designs, the corridor space is often less than 5 feet across, and ceilings are low.
In game terms, there's often no room to swing a blade. Slashing sword attacks and similar strikes will be at a severe disadvantage. Dungeons favor thrusting weapons.
And in a chamber, the roof is rarely flat. It's likely a vault or an arch. Even so, there's often not a lot of headspace, and you're unlikely to be able to manage a down strike, or shoot an arrow or thrown spear very far without hitting the ceiling.
In game, missile attacks are limited to close range.
And stairs
Towers are great settings, but stairs, especially circular stairs tend to be extremely steep and narrow. Climbing is tiring, and the risk of putting a foot wrong on the narrow steps is considerable if you're under pressure: attacked or in a hurry.
In game, climbing stairs under pressure should require a dexterity or fatigue check. And defenders have an advantage striking down. Consider also how missiles and objects thrown down stairs can create serious hazards!
Darkness and light sources
The actual underground is absolutely dark. Without light, players are effectively blind, and almost blind a very short distance from the entrance.
In actual pre-modern contexts, lights such as lanterns, candles, lamps, rushlights, and torches, only effectively illuminate a few paces in advance, or a small chamber. And if they're knocked out of your hand, they're likely to go out instrantly.
On the other hand, even a dim light is visible to observers from a long way away – effectively in line of sight.
This means that the most effective scout will move forward in the dark towards an unknown light source, rather like Bilbo Baggins sneaking up on the trolls' bonfire.
Used well, the effects of undeground darkness and low light can help create a tense, engaging situation for players, and keep them on edge as they explore unknown spaces. But many games, including current editions of D&D, forestall these options by making many character backgrounds adept at seeing in the dark, or with magical or other options for lighting available throughout the dungeon complex. If you like your encounters to effectively play out in well-lit arenas, so be it. But to add back some of the danger and uncertainty of low-light conditions, consider restricting those "night vision" abilities to situations where there is at least minimal incidental light, or applying other restrictions (for instance, low-light vision is greyscale only).
And while we're at it, lamp oil is not the basis for some kind of medieval Molotov cocktail. Fire, on the other hand, is potentially a great hazard in structures built with wood, with many wall-hangings and flammable materials.
