You Must Be This Tall to Enter

One way to control the plot of an RPG is to place barriers in the hero's path. A destination may be unreachable without access to water transport because the main character has Super Drowning Skills. You may need to become immune to fire before moving on because the next level is set in a Fire and Brimstone Hell environment. Perhaps you need to enlist the help of a specific NPC, or acquire a specific item, in order to get past the annoying guard.

Sometimes, the item or ability or whatever isn't needed again. But in other cases, it becomes a key part of the main character's inventory, and gets used all the time. When this happens, the rest of the game effectively has a You Must Be This Tall to Enter sign attached to it.

In real life, these signs are used on amusement park rides and suchlike, because it's easier to design safety restraints for a ride if you know everyone on the ride is big enough to wear them safely. Almost everything else about the riders is irrelevant, as long as they are tall enough.

In many games, something similar happens. You can't get to the other island without a ship, so any plot event that happens on the other island can assume you have a ship. Thus, the game developers suddenly start throwing quests at you that require you to hop back and forth between islands like a bored tourist, and continue to do so for the rest of the game. In extreme cases, the entire game dynamic may change after you obtain this one critical item or ability, because you are now well-equipped/powerful enough to handle challenges that would have been insurmountable before.

See also: Broken Bridge, But Thou Must. For something that requires you to be at a certain level to proceed before it kills you, see Beef Gate. When used carelessly, this can result in Cardboard Obstacles.

'This is not'' an instance where a character in a show cannot get on a ride because they are a certain height. This is for the figurative not being tall enough in terms of level or other requirements (needing a certain of a certain item to enter also may be an example) in order pass into an area.'''

Action Adventure Game

 * In the PC version of Harry Potter and the Chamber of Secrets, you cannot get into some game areas until you are taught specific spells in class.
 * You cannot use Spongify pads to jump until you are taught the Spongify spell, but various Spongify pads outside the castle are visible throughout the game.
 * A literal example of this trope is the arcade game Bubbles. In this game you control a scrubbing bubble in a sink, and advance to the next level by sending your bubble down the drain. But you have to grow your bubble big enough before you can go down the drain; if the bubble is too small, it'll pop.
 * In The Legend of Zelda Ocarina of Time, there are several items that act as You Must Be This Tall to Enter signs. For example, you can't get into the Forest Temple without a Hookshot. As soon as you get inside the Forest Temple, and for the rest of the game thereafter, you will see Hookshot targets all over the place. Many important puzzles are insoluble without the Hookshot. Good thing you never ran into one of those as a kid, before you got the Hookshot, huh? Let's not forget that You Must Be This Tall To Wield The Master Sword...Literally.
 * And as a series hallmark, every dungeon in the series will have puzzles that can't be solved without the item found in the respective dungeon.
 * It's worth mentioning that in the original The Legend of Zelda, Link needed a minimum number of Heart Containers to get the stronger swords.
 * In The Legend of Zelda a Link To T He Past, rocks, stakes, and similar barriers exist all over the world. Most dungeons require you to use the treasure to pass some puzzle or barrier within it. And of course swimming skills are required to visit places all over the series.
 * Let's just call this like it is - the entire Zelda series is littered with this.
 * Nearly every single boss is an example of this trope in the Zelda series. Any time you find a new item in a dungeon that could possibly be used in combat, expect to use it against that dungeon's boss.
 * Used very frequently in Okami. There are places very early on in the game that require some of the final Brush Techniques to enter.
 * Metroidvania games are built on this trope. High jumps (You Must Jump This High To Enter), narrow passages (You Must Be This Small To Enter), and especially the color-coded doors of most Metroid games.
 * In a Lampshade Hanging fashion, said games also force you to use the item/powerup you just got, merely to escape that room. Just got the High Jump? Why, look there. You can only get out of the room by using the High-Jump. Unless you do some Sequence Breaking techniques, that is.
 * Certain Metroidvania-style games have areas that must be entered by double-jumping. Usually, those areas are full of ledges (and Bottomless Pits) that can only be travelled by double-jumping.
 * Applies not to you, but to your minions, in Overlord. Can't pass fire without red minions, or poison without green ones! You can go through water, but only blue minions can safely follow you, and if there's a gate-opening wheel on the other side you're just too lazy to turn it yourself.
 * Sounds suspiciously identical to Pikmin.
 * Castlevania games generally tend to give you the double jump fairly early - you'll need it constantly for the rest of the game. However, many of the upgrades are also used only once or twice.

Eastern RPG

 * In the Dragonball Z Legacy of Goku 2 and Buu's Fury games for the Game Boy Advance, certain areas can only be entered by certain characters having reached certain levels. This is evidenced by the number they must be at, bearing a color associated with the proper character (blue for Vegeta, green for Piccolo, orange for Goku, etc.).
 * Final Fantasy III has a `you must be this short to enter' feature -- certain areas are accessibly only through tiny doorways, only available once a member of the party has the `mini' spell.
 * Mario & Luigi: Superstar Saga also has some areas that are blocked off or otherwise inaccessible until the brothers have learned some ability.
 * This gets particularly annoying in later game Mario & Luigi: Bowser's Inside Story, where the Bros are simply remembering moves they knew in previous games but mysteriously forgot. They don't remember the first time you find an appropriate obstacle, oh no; you have to wait until the plot gets to the right point, and then backtrack later.
 * In Toad Town there's a shop which won't sell the Bros gear until their rank (based on experience level) is high enough. Leaving aside the usual question of why you're being charged for equipment when you're on a mission to save the Princess, let's think about that for a second. If Mario and Luigi aren't high enough level to shop here, who the hell in the Mushroom Kingdom is?
 * Paper Mario has this with the obstacles cleared by partners' abilities.
 * In each of the Pokémon games, there are certain cities and localities you can only reach once you have a Pokémon that knows the move Surf...and once you have the Gym badge that allows you to use it outside of battle.
 * A mainstay of the Wild Arms series where characters use tools to pass certain obstacles. The location where they are introduced often requires extensive use of the tool which was never required before that point.
 * Phantasy Star III makes use of caverns to transport your party between various worlds. To keep you from advancing too far too soon, several of them are locked and require specific gemstones to enter; you acquire those either from Inexplicable Treasure Chests or from recruited party members.

First Person Shooter

 * Half Life 2: Episode One used this relatively early. To make sure you have a gun, the door has a padlock just out of reach. Mainly done to ensure people picked up the gun right by the door, since it is a bit dark in there, and it could go unseen. The commentary reveals that the dev team calls this a "gate".
 * In Hellfire (an expansion pack to the original Diablo), you cannot reach the insect hive until the farmer character knows you well enough to talk to you about his problems (and then to give you the explosives you need to create an entrance to the hive).

Platformer

 * Banjo-Tooie does this to limit progress through the Hub Level. You need to have learned Grip Grab to get into Plateau, Fire Eggs to get into Pine Grove, Split Up to get into Cliff Top, Talon Torpedo to get into Wasteland, Springy Step Shoes to get into Quagmire, and Claw Clamber Boots to enter the Cauldron Keep. Even the in-game cheats don't get around these requirements.
 * The Zubba hive in Cloud Cuckooland presents a variation. You're not allowed in unless you demonstrate you can shoot a target 20 times in 10 seconds, which effectively means You Must Be A Bee To Enter.
 * In the second level of Little Nemo the Dream Master, Oompi says, "You're going to need the lizard's help to get through this next area." This alludes to a passage that is otherwise too narrow for Nemo to enter.

Puzzle Game

 * This is literally the case in the Katamari Damacy series; the larger your katamari becomes, the larger the stuff you can roll up. Levels frequently have barriers with signs on it stating a size number, which you can't roll up (or thus explore beyond them) until your katamari crosses that threshold.

Roguelike

 * To descend into the deepest parts of the main dungeon in Ancient Domains of Mystery, the player needs to pass a wall of fire blocking the down staircase, which requires an artifact called the Chaos Orb of Elemental Fire that is also required in the endgame. And it's just as well that the wall forces the player to be that tall get the Orb, because otherwise no sane person would enter the Tower of Eternal Flames where the Orb, massive equipment destruction, and rapid immolation for the newbie player resides.
 * Earlier versions of Nethack had an endgame set in Hell, and if you entered it without Fire Resistance you were instantly burnt to a crisp. (These days, getting anywhere near the endgame without a bucketload of resistances is merely mostly suicidal.) Somewhat subverted in that there are plenty of ways to get fire resistance before this point.

Survival Horror

 * This happens all the time in various Resident Evil games, often with either a crank or valve handle.
 * The service elevator in Silent Hill 2's hotel only allows one person, not including baggage.

Western RPG

 * Different obstacles in the Dreaming in Dragon Age each require different alternate forms, obtained from lost souls somewhere in the dreamscape. The rat can go through rat holes, the spirit can go through portals, the golem can smash large doors, and the, er, burning corpse can walk through fire.
 * In Dungeons & Dragons: Warriors of the Eternal Sun, your party are unable to explore a particularly volcano-y cave without every member wearing Rings of Fire Resistance. Aside from their obvious advantages involving the resistance of fire, these items are only really necessary for this part. Take a ring off and your character will slowly burn to death.
 * Lands of Lore 2: Guardians of Destiny contains lots of areas that can only be accessed if the hero is in the form of a giant beast (allowing him to shove obstacles out of his way) or a tiny lizard (allowing him to slip through cracks). The hard part about this is that, for a good chunk of the game, you have absolutely no control over when he transforms, or which form he changes into if and when he does.
 * Played to some degree in The Elder Scrolls III: Morrowind, where You Must Levitate To Enter a Telvanni mage tower. You can find, however, plenty of levitation potions around, you can make your own if you need to, and the main quest also gives you an enchanted item with a Levitate spell burned on it.