The room in a video game where the boss and the player exchange blows. Typically features an imposing entrance door, and decor fitting the boss villain's Motif.
The appearance of a boss room will typically have the following characteristics:
Classic boss rooms are often ridiculously large and can be picked out immediately because of their emptiness and wide-openness, especially in a First-Person Shooter. Usually, this will be because the boss itself is also a ridiculously large creature or machine and needs room to move, and because the player needs lots of room to dodge the boss' attacks and to run away if the boss gets too close.
A newer variant of the boss room is the "maze of obstacles" room, often something like a cluttered office or a system of sewer tunnels. These cause the boss battle to become a hide-and-seek or a chase sequence rather than the typical shoot-and-dodge battle.
On occasion, the room may darken ominously upon entry, or the composition of the floor and walls may vary noticeably, and the Genre Blind heroes may not notice, or mind.
And sometimes there is a large object or large, mostly simple or blank wall, that will be broken by the boss on entry, or be the exit wall the nicer Ghost Butler will kindly open for you by the dying boss shattering it with weight.
The boss may be waiting for you in plain sight, or you may have to search before realizing that He Was Right There All Along.
Exit is typically not allowed until the Boss Battle is over, after which one may leave the way one came, exit through an alternate door, or be teleported out, through either Phlebotinum or plot device.
It's considered tradition in the Castlevania games for Dracula to be fought in the highest room of the tallest tower in the game. As of Akumajou Dracula for the Sharp X68000, it's also typical of him to take a sip from a wineglass of blood and throw it dramatically at the player.
In many of the 2D games, every boss room has a wide, ornamented automatic door that glows cyan blue. It also slides into the ceiling instead of opening like every other normal door. The room leading up to this door usually holds two candles where you can refill hearts for your sub-weapons.
Fights against Bowser in the Super Mario Bros. games usually take place in a lava-filled room, or one with a bottomless pit. Normally these hazards are how he meets his demise, except in Super Mario 64, where he'll simply jump back onto the platform after being tossed off it.
The level itself is usually also a clear hint that you are about to fight a boss. There are the typical castles in the 2D games (often with a big door leading into the Boss Room), the rather surreal Bowser levels in Super Mario 64 and Galaxy... although the 3D games at least spread bosses among the normal star objectives aswell. Depending on the star, it can be more or less obvious from the start.
In Galaxy and its sequel, we have boss planets instead. Bowser's planet (or Sun in the final level, where he is fought on three planets, including said Sun) in the first game resembles a metal sphere filled with lava and surrounded by six blue glass panels allowing the lava to come out of that get smashed apart when Bowser steps on them, setting his tail on fire, and allowing you to defeat him, while in the sequel it resembles a burnt rock planet in which a giant Bowser will try to punch the planet and send meteorites crashing down on it (you Ground Pound the meteorites back at Bowser in order to defeat him).
Every boss in ActRaiser fights on an open area at least as large as the screen (depending on the boss). Most of these areas are symmetrical and provide with platforms much needed to avoid their blows.
Subverted in Silent Hill 2, where you find a really big room that seems perfect for a boss room. All you have to do is to solve a puzzle, but you feel like you're going to get jumped by one. Also, many of the actual boss rooms in the game are incredibly small, with barely any room to maneuver. For example, the stairwell in which the first Pyramid Head battle is fought, and the room where the Abstract Daddy is fought.
In Fable I, the final boss battle takes place in the aptly-named Chamber of Fate, which is a massive domed room with gothic architecture which has frescoes adorned across the walls of your exploits in Albion, added as you completed each goal.
An earlier boss battle against the bandit leader Twinblade takes places in a circular clearing, with lesser bandits forming an edge.
Zelda games make it painfully obvious with a huge door that is also locked, often with a big, mean looking, distinct keyhole that needs the dungeon's Big Key (or Master Key) to be opened, even though getting the Key can sometimes be pretty easy compared to actually getting to said door. To make it even more obvious, the room is marked with a skull once you pick up the compass.
The original The Legend of Zelda didn't have the door to the boss room be any different from any other; the only clue the boss was in there was the sound of his roars.
In The Legend of Zelda: Skyward Sword, one boss subverts it entirely. When you enter the room locked by the huge lock, you're chased out of the room by the boss, who instead faces you in the hub area of the dungeon.
Final Fantasy XI has "Burning Circles", battle arenas where special mission and quest related fights take place. These usually consist of a long walkway leading up to an open circular area, although a few are designed differently, such as the Elemental Cloisters. Perhaps the most unique Burning Circle is the Shrouded Maw, which consists of a group of floor panels suspended above a pit filled with high level monsters. Players must watch out for collapsing panels as well as the boss's knock-back attacks.
Parodied in The Adventures of Dr. McNinja, where Ronald McDonald is found in a featureless black room. He explains that this is the room video game bosses hang out in until their minions are beaten.
In Doom, the secret level for Episode 3 looks exactly like the map for Mission 1, until you get to the end of the level, and step on what you think is the exit teleporter. A wall drops instead, exposing a Cyberdemon (the really tough boss of Episode 2, who is also fought in a similar area), and the key card you need to acquire is behind him, so you either have to kill him or run well enough to get past him, steal the key and leave.
While the Cyberdemon here is still every bit as powerful as the encounter in Episode 2, this "rematch" can potentially be made easier due to the fact that Episode 3 gives you access to theBFG. But said BFG is hidden away in the levels before, it's quite possible to fight him again without it. In which case...
Not to mention the eighth mission of each episode.
Doom II: Hell on Earth has MAP20: Gotcha!, with an extremely large room with two pillars in the center. Upon entering the room, the pillars rise up to reveal both a Cyberdemon and a Spider Mastermind. Played with in that it's actually set up for you to deliberately invoke a battle between the two, after which you can kill the weakened victor.
The boss battles in the Prince of Persia: Sands of Time series mostly take place in huge halls, elevated platforms, battle arenas or the top of the tallest tower of Babylon.
Prince of Persia: Warrior Within has a large chamber behind the throne room that is entirely unlike any other room in the game — a giant, flat stone table, in the middle of a stream running through a cave, with no walls or other climbable spots. Three separate boss fights take place here.
The Last Remnant doesn't feature specific rooms for bosses, but you can bet the dead end on the map the quest is directing you towards will have a boss in it. Also, there are world map locations that take you straight to a battle, and are considered Boss Fights.
Robot Masters in Mega Man even get pre-Boss Room hallways with ominously locking gates. The "Dr. W" sign can be seen outside many of these hallways when Wily's not hiding behind someone else. The pre-Boss Room hallway was originally designed as an obvious checkpoint, so that the player would be able to ready their special weapons and such. There were even enemies to get weapon energy from, if needed. However, starting with 2, all the pre-Boss Room hallways became very featureless, and are typically viewed as a brief though unnecessary breather before the boss. The original game was supposed to be on disk, and the long pre-boss chamber was to allow the game time to spin up and load the data for the boss battle. When they switched to cartridges this was no longer necessary so they did away with the enemies in subsequent games.
In Digimon World 2, at least, you can always tell when a boss room will be coming up by a strip of 6 or so specialty terrain features in the confined hallspace. In fact, even the NPCs notice this, because in the tutorial/training mission, Zudokorn, your superior, speaks up about the boss digimon coming up soon — though he doesn't explain it, he says it's "just a hunch".
Jet Force Gemini wants to make absolutely sure the player knows when a boss fight is coming. Every boss in the game is preceded by an empty room stocked to the rafters with health and ammunition for all the player's guns, and the following room is always a massive, seemingly-empty chamber, which is perfect considering the average boss is about fifty feet tall.
In an odd non-video game example, Spider-Man's first confrontation with Shikata, in the MTV Spider-Man: The New Animated Series episode "Sword of Shikata", played out exactly like a boss confrontation of this type, down to being locked inside a wide-open space for a moment, only for the adversary to dramatically reveal themselves, red-backlit, from behind a rising door before they get down to fisticuffs
Pokémon both plays this trope straight, and averts it. On one hand, a majority of boss battles take place at the end of Gyms, or at the ends of dungeons, or a series of rooms and corridors for the Elite Four. Most legendary Pokémon stand on top of raised platforms, politely waiting for you to approach them. However battles against The Rival and the roaming legendaries are scripted to come out of nowhere.
The first game hinted at by architectural features in the room before; themed with architectural features. However, they're only one room big, so it's not easy to tell it if you had a map (which you don't really).
Metroid 2: If you count each of the metroids as a boss, then the game practically has no boss rooms, except for the last boss.
Super Metroid, Metroid Zero Mission, Metroid Fusion: Bosses are indicated on the map by a monster's image. After the boss is defeated, that image is replaced by a big X. Most boss rooms are also 2 units by 2 units large, and are also indicated by a large eye-creature covering the door that would shoot Eye Beams at the player.
Metroid Prime: When playing Metroid Prime, you can usually expect a boss fight if a room has the following properties: (1) it's been indicated as where you need to go, (2) it is large, (3) it has few platforming features in it.
The Prime series' boss rooms also tend to be circular in shape.
La-Mulana: Boss rooms may (Dimensional Corridor, Confusion Gate, Spring of the Sky) or may not (Mausoleum of the Giants, Twin Labyrinths) be obvious before you reveal the boss's ankh. But once you reveal the ankh (which has to be broken using an ankh jewel in order to reveal the boss), simply going into the boss's room causes the music to change to the pre-Boss Battle music ("Requiem"). The room will then be devoid of enemies (for the moment), but will have a noticeable ankh somewhere.
The "huge space to fight huge creature" aspect is played straight in the Baldur's Gate series, in which dragon lairs are usually quite telltale.
World of Warcraft has many examples of boss rooms, although some of them are huge rooms with tons of minor mooks that need to be dealt with beforehand.
Perhaps the purest example is the Eye of Eternity, which is an entire boss realm. The raid consists of nothing more than a large circular platform floating alone in space that players can reach via teleportation to fight Malygos. The location is used again in the Dragon Soul raid to fight a different boss.
In zOMG, the only dedicated boss rooms are the Stone Coatl's Throne Room, and Sea Lab X, where the Mechlab Bot is kept. Most other bosses appear on the main map, though they spawn in specially designated squares. These squares are usually somewhat spacious, may be guarded by some weaker Animated, and are often blocked off from the rest of the map in some way to prevent the boss from escaping. (Though there have been many instances of a boss escaping.) The Boss Map may also feature a chest as an additional reward. Notably, if you see a big square with very little decor, it's probably a boss square.
Metal Gear Solid did the 'maze of obstacles' rooms before it was cool, with the fights between the Ninja (which took place in Otacon's cluttered office) and Psycho Mantis's boss fight (in a well-furnished office for the base's Commander).
Dead Space: Take a look at the map, see any unusually humongous rooms on this level?
The Elder Scrolls III: Morrowind: The room where you can actually fight the Big Bad is about average for most dungeon chambers — rather cramped considering his size and reach — but it is symmetrical. The room where you finish him off is the massive interior of a volcano.
The Boss Room of Silent Hill 4 is large and symmetrical, but most of the space is taken up with a dramatic centerpiece, leaving the net combat effect of battling in a curved corridor.
Many rooms in Final Fantasy XII fit this trope, including the Sochen Cave Palace where you fight the five incredibly annoying onion creatures.
In the Mega Man Legends series the boss battles would always be telegraphed by a cutscene. In one case in the second game the cutscene involves a electric barricade suddenly materializing behind Mega Man. In the first game the island reaverbot boss comes seemingly out of nowhere and the trope is subverted as the room is actually much smaller than most of the other rooms in the dungeon.
The ones in Iji are quite obvious, except Krotera's. It's not like you wouldn't know anyway, though, since all of them are after the end of the level proper. Except Asha.
The room where you fight the Joustasaurus in Backyard Skateboarding has all the qualities of a boss room...except it is small.
Partially averted in the Armada-based Transformers game. Though a Boss Fight may start in a certain area, in can move outside if that's the way things go. This is particularly evidenced in the Pyramid-top battle against Cyclonus, who will most likely throw you off within the first thirty seconds of the battle.
In Magical Doropie, boss rooms had a door closing at the top and an "A" symbol similar to Wily's "W".
In the first Ys games, boss rooms lie behind significant-looking double doors and are shaped to fit the player's entire viewing area. They are the perfect place to take a little health-regenerating rest after the boss has been defeated.