I may or may not finish these series. I've lost and found them dozens of times before publishing. Anyways, here you go.

Goal, task, plot. The stuff without which a level is not a level

Before creating a level, you need to be clear about three things:

Level Purpose: Why are you creating this level?

to introduce new mechanics;
to test the player;
to tell an important episode of the plot;
to introduce a new enemy;
to teach the player something;
etc.

A goal is a practical reason for creating a level and is not related to the plot or aesthetics.

The goal "just to be here" and "to stretch to N levels" is a bad goal. It is easy to see such levels for players, and they give little fun.

Next, the task of the level: how do you and the player complete the goal?

for the mechanics, you will need to demonstrate it with something: through NPCs, hints, or by creating artificial conditions that result in obvious actions.
to test a player, you may need to limit the time, dramatically increase the difficulty, make the player think strategically or tactically, or combine all the difficulties at once.
for the plot — how not to turn the level into a cut-scene?
for new opponents, you can make an isolated arena that provides the ability to think before acting and to recover. The arena should be without other enemies.

To train the player, see the options with mechanics.

Are there any additional tasks, e.g. arising from mechanics and game loops? For example:

train a player new tricks by using existing assets;
refresh existing knowledge and polish skills;
increase the power of a player in sense of mechanics (basically level up a character);
give or take away something in terms of gameplay (e.g. a previously essential ability);
offer side quests;
etc.

These sub-tasks must not replace the core task of a level.

Finally, the plot will truly link the levels together and add meaning to the location so that it doesn't become a filler.

In the early stages of level creation, it is not necessary to immediately think about the connection with other levels in the story, but each level can tell its own mini-story. For example, through storytelling in the environment, or by staging small self-encapsulated stories: about the world, the game character, NPCs, or a nemesis.

Examples

StormCross is a sea voyage simulator, where you need to save a dying colony from fever. There are three levels + a small ending. The first two levels are better worked out.

You can firstly complete this game, then return here — it is not so long.

In the very first level, there are four components:

Goal: immerse the player in the atmosphere of a naval journey.
Task: to separate the concepts of "home" and "sea" and to show the vastness and wilderness of the sea.
This is achieved by firstly showing the player a bay with a settlement by it, enclosed with mountains, and then showing a vast sea that spans from edge to edge of the screen, where you can no longer see the ground.
Later, you find tiny patches of ground, with scarce resources and hazardous weather.
Subtask: teach how to collect resources, introduce a player to the first opponents, teach how to control the ship.
Plot: the ship leaves its home bay and goes to rescue the colonists at the first island.

In the second level:

Goal: show the crew members of the ship as living beings.
The player is guaranteed to lose a portion of its crew members, at least because of lack of food.
Task: limit resources and increase the number of dangers. Evoke a feeling of anxiety in the player.
The player is ambushed by pirates in a relatively open area but then has to battle through tiny corridors with spikes, reefs, and steep mountains. Bolts of lightning coming from dark moving clouds pose an omnipresent threat.
Plot: the ship enters a storm zone and finds a pirate base among the cliffs.

The third level was made in a hurry and is not that well-planned. Anyways, in the third level, the final test is set for the player as large pirate ships appear in the center of the storm. After the battle, the end the player returns to the bay and relieves the tension. Music helps with it a lot.

My arcade shooter Bitmaster has only two levels - an interesting main arena and a terrible boss battle zone. At the arena:

Goal: create a space for core gameplay that will maintain the proper level of action throughout the whole session.
Task: build a geometry that will make the gameplay more difficult with the appearance of new types of opponents, sometimes creating sticky situations for the player.
Storyline: The level is an abandoned software production facility where one can gather the energy needed to travel over long distances.

The arena is divided into four big parts, each having different effects on it, either negative or positive. These buffs and debuffs could also affect the whole map, which was a bad decision, as it weakened the whole point of moving through the map quickly to get the needed buffs.

Since the plot in Bitmaster was made after the appearance of the game in the Steam greenlight, the conceptual implementation of this arena was an afterthought and is a failure. It could benefit a lot with decorative touches and additional meshes that would make this arena truly an abandoned facility. Just by looking at it now, there won't be such thoughts.

ToC

Goal, task, plot. The stuff without which a level is not a level

Examples

Next in the series: The process of making a level →