The level design must always be tied to the game's story. The story, the theme, the environment and the game should all resonate with each other. In the TV Show CSI all episodes are about criminal investigations. Every crime leaves behind traces and these traces must be found by the forensics people to be analysed and for the story of the crime to be later reconstructed from all the evidences and clues gathered. Games should also employ clues and traces in storytelling. If the game is about a civil war the environment should transmit the feeling of a war. If a city is rich and safe it should be clean and feel safe. A mistake that games shouldn't make is to mismatch the environment and the event, such as telling that there was a shooting case while the environment is clean of any shooting marks anywhere.

It's important to note that not every player is going to read the environment in the same way. In many of CSI's episodes the first interpretation that the investigators have of what happened and who did what changes over the course of the episode, as more clues and pieces of information are gathered. By the end of the episode the story of what happened and who did what is completely different from what the viewer was expecting at the beginning. In games that may be intentional, as in the case of a villain who is intentionally manipulating the player. The problem arise when the developer or designer's intention isn't clearly communicated, with the players not grasping the idea that was to be given. The same is true for challenges and obstacles.

There is a technological challenge in regards to telling stories. If the environment is clean at one time and dirty at another, it's the technology that allows to change the textures, lighting, move objects, to tell a story. At the same time, technology itself isn't something that prevents a story from being told. For example, a place is flooded by rain. To simulate water flooding the place while keeping the physics as close as possible to the real world is too heavy for a home computer to process. Even on cinema it's a very demanding task. Nevertheless, nothing prevents an environment to be made under two different states, one dry and another wet. In movies a creative solution to depict scenes of destruction is to build miniature versions of the environment. It is this type of creativity that is required to tell stories through the game's environment. In the case of CSI the crime has already happened, what is left are the evidences. In a game it may be unfeasible to simulate a flood in real time, but the environment itself may tell the player about the flood by having evidences of having been flooded.

There are many different ways to marry the theme with the environment and the story. Whichever mode is chosen there must be coherence and cohesion. To place many burned corpses in a clean room with no burn marks anywhere feels detached.  A humble and poor character living in a large and luxurious mansion doesn't make sense. The architecture of the place itself has to to be coherent in itself. A bridge can have any architectural style, but it has to convey a sense of realism by being strong and stable. It feels detached to have a bridge that is clean, beautiful, in the middle of a environment that conveys destruction and has no explanation for the bridge to be disconnected from its surroundings.

There should also be a careful research to prevent incoherences from happening. If a character follows a certain religion, the clothes and environment of said character must match that religion. Sometimes the architecture of a place mismatches the correct religion because there was a mistake made during the research. An example of that mistake is the swastika. The swastika is well known to represent the nazi forces, but it also used with a different meaning in Jainism and Buddhism. The difference is that one swastika is mirrored and the other is not. If a game has references to Chinese architecture those references should be Chinese, not some randomly chosen features from Thailand or Philippines. Thailand and Philippines are Asian countries but they are very different from China.

Linear x non-linear

The level design can be linear or non-linear. By extension, the whole game can be linear or non-linear. A linear level is easier and more intuitive to make because the events are sequential and easier to follow. A linear game is also easier to make for the same reason. Having a straight timeline is much easier to follow compared to having loops, parallel timelines or time  travel.

In a linear level the obstacles and challenges follow a fixed sequence that the player always traverse in the same order. This way the points A, B and C can't be skipped and the player has to complete them in the order A-B-C. It's much easier to build a level following a linear order because when the player reaches B, then objective A has been completed and when it reaches C, A and B have been completed before. It's much easier to predict the player's actions and progression in a linear fashion. It's more natural for the difficulty to escalate from easy to hard, from A to C.

The areas of a level doesn't need to follow the order A-B-C. The areas can be reviewed after some trigger unblocks areas that were previously blocked. Or after some inactive object is activated in a later stage. Exploration of environments is greatly improved by modifying things that the player has already seen in a previous area. The level itself is still linear with objectives following a specific order, but allowing the player to backtrack and explore can be engaging if there are new things to discover.

The non-linear case is more complicated. The level may have a door that requires three keys, A, B and C. The order doesn't matter. The player can go after A, B or C in any order. The difficulty of each objective, A, B or C can be equal to each other or different from one to another. There isn't a rule for this and nothing prevents the player from completing one before or after another. The objectives have no dependency with each other. There could be a dependency relationship between objectives but that creates a complex web that is hard to track. Suppose that there are two objectives and completing one changes some detail that affects the other. This leads to two possible routes with the same outcome. With more than two objectives it becomes more and more complex to track down all possible combinations.

A way to combine linear and non-linear is to have a primary structure that is linear. Suppose a game that has 5 parts. Each part is further subdivided into 3 chapters for a total of 15 levels. The 5 parts are always completed in a linear sequence. But the chapters in each part can be completed in any order. In terms of storytelling, the story follows a sequential order and is split into 5 parts. But there are subdivisions that have no preferred order. Since every story has a beginning, a middle and an end, it's hard for the levels in a game to be placed in a non-linear order. The story may have non-linear parts, but there must be a linear underlying structure being followed.

Cultural references

This is probably one of the most commonly used techniques in TV shows and movies. Lots of cultural references. There is a matter of prejudice and unintentional cross references that may be offensive to some groups, but I'm not discussing that aspect here. Mark Rosewater, from Wizards of the Coast, the company behind Magic the Gathering, says that one of the most powerful assets that they have at their disposal is to take advantage of cultural references. When players find them in the game they are more likely to have a deeper emotional attachment to that game. Take a look at superheroes movies such as Avengers, Justice League, Superman and so on. The writers don't create scripts from nothing or out of their own views. They have to write stories and scenes that reference to the comics, because there is a risk associated in ruining one or more characters, settings and the outcome of that could be a disaster. An outcry of fans publishing negative reviews for example. How many movies or series failed because of that?

That's the problem with remaking games. Sometimes you try to fix what is not broken and end up with a worse product. I don't have experience with that and because of this I cannot go on with further discussions. I don't have an answer to when a reference should or shouldn't be made. Or what references to make. If you look at the Duke Nukem 3D game it has many cultural references to movies and other games spread across all levels. From a personal viewpoint I'd argue that sometimes people can fell the need to be original. They want uniqueness and that means not using cultural references. Taking Mark Rosewater's own 20 lessons about game design, I'd say that there is a matter of ego behind the desire to feel original. Maybe a different way to face this matter is to think that the originality isn't always about creating something completely new, but in viewing something that already exists from a different perspective. There is a ton of source material to draw ideas from, but each person have its own unique way to interpret it.

I should say that there is a difference between plagiarism and an homage. The first can very well be a crime while the second is not. We all draw inspiration from things that matter to us because they mean something important. Whether we are copying it or adapting ideas it's some sort of grey area because we are dipping into the realm of copyrights.

Scripted events

Any event that is expected to happen when certain conditions are meet. If it doesn't happen we have a bug. Most games have scripted events in some form. They are meant to make the player experience something or just be sure that something happens exactly as it was intended to happen. In the original doom there were boss fights that could not be bypassed in any way. In modern games there are cinematic sequences or dialogues that always happen in the same order no matter what. Storytelling relies heavily on this.

Level design is directly affected by scripted events because the level has to be designed around it. Suppose that a monster is going to attack the player. How is that monster going to be activated or how is it going to come in? We can have a door that is opened by some character, the monster breaking through a wall / floor / ceiling, the monster being teleported in from elsewhere, the monster suddenly awakening, etc. Whichever way is chosen by the director the level design has to account for it.

The hardest problems to solve arise when we have dynamic events that interact with physics and/or AI. In such scenarios we have a certain degree of unpredictability that can be really hard to solve. A pretty simple example: suppose that you have to throw a grenade, if the wind is too strong and the grenade has physics to be pushed by the wind, we may miss the target. This works for the AI too because they may perform actions that don't produce the expected outcomes.

Another aspect that can't be overlooked is that players are going to be mad or frustrated if whatever scripted event happens, it forces the players into a direction that they didn't want to or could clearly see that it wasn't the best action to take in that moment. You have to be careful about what you want the players to experience. How they are going to perceive it cannot be disregarded.

This brings us to the question: when to trigger a scripted event? I don't have experience directing and I can't really tell. From a player's perspective the two most adopted times are the beginning and the end of each level. Now for anytime in the middle we have to be careful because excessive scripting leads to the player watching the game rather than playing it. Not good. There is also a matter of atmosphere, immersion and decision making. If the player has a lot of influence in the game's world and then, suddenly, the game's director takes over and forces something to happen by taking control of the player's character. That's not a good idea. Another reason to have scripted events is pace. There may be moments to insert jokes for example. Or let the player breath before something that requires energy and/or concentration. A third reason is hardware limitation. There is a limit on how fast computers can load content and often we are forced to slow down the gameplay to give time for the computer to load what we need to the system's memory. That's when a scripted event takes place to hide the loading screen.

From the games that I've played here is a list of times to trigger a scripted event:

• The beginning and end of each level;

• The beginning and end of boss battles;

• In the middle of a boss battle when the boss has multiple stages;

• When the player transitions into a building;

• When the player comes out of a building;

• When the player enters a new room or area;

• When the player leaves a room or area;

• When the player opens a door or before opening it;

• When the player pulls a leaver, presses a button or activates a switch;

• When the player activates a trap;

• When the player completes some objective or mission;

• When the player enters some transport system (metro, car, ship, etc);

• Scary moments such as birds flying off a corner, rats running, debris falling down, surprise attacks, smoke screens, etc;

• When the player meets a new character in the game's plot
  
  

The previous question unfolds into its opposite question: When to not trigger a scripted event? From what I know, the most common time would be in the middle of the action. If the player has all their attention focused on defeating a boss, it'd be a bad idea to trigger something that is unrelated to that boss. For example: in Max Payne 1 the last objective is to prevent the last boss from escaping. The player has to prevent the boss' helicopter from flying away. It wouldn't make much sense to have an airplane crashing down elsewhere, driving the player's attention away from their objective. I could cite one movie scene to exemplify. In "Ip Man" there is a fight between Ip Man and Jin Shanzhao. In the middle of it Ip's son interrupts the fight, happily riding his child tricicle in the middle of the two fighters. The two fighters and the audience have their attention briefly shifted towards the child, before the fight resumes. Could the fight have continued while the child was riding his tricicle? This is not what the film's director would've had wanted. In case you don't know the movie or didn't remember the scene, here is the link: Ip Man vs Jin Shanzhao 

Is it wrong to interrupt the action with a scripted event? I don't have professional experience but I wouldn't state that it's always wrong to do so. Taking the previously mentioned fighting sequence as an example, the director probably knew what he was doing by interrupting the fight the way he did. In games it's even common for that to happen. During boss fights there may be programmed times when the battle is intentionally interrupted to give the player time to run, time to recover, to play some dialogue or something. Maybe some allies are schedulled to jump in and join the action. Some battles are scripted and not meant to happen without cuts and in one shot. Path of Exile for example. In the Izaro's Labyrinth there is a boss fight that is split into three acts. The player encounters the boss twice before facing it for the final battle. It's one large battle subdivided into three acts.