Chris Johnson - Game Designer

“The Rat Nest” – Custom Half-Life 2 Level

“The Rat Nest”

A thrilling combat mission made as a custom level for Half-Life 2

Developed By: Chris Johnson

Engine/Framework: Source Engine, Hammer World Editor

Platform: PC

Type: Linear Singleplayer First-Person Shooter

View the full Level Design Document (LDD) here

Download and play this level here


“The Rat Nest” is a custom singleplayer level for Half-Life 2. Originally created as a project for a Level Design II Course at Champlain College, this level was created in Hammer World Editor using only assets from Half-Life 2. In the level, the player character (Gordon Freeman) must rendezvous with the resistance at a safehouse within “The Rat Nest,” a sprawling urban apartment complex inspired by the real-life Kowloon Walled City. The player has two battles with Combine before reaching the safehouse and meeting Alyx, the resistance contact. Alyx gives the player a new secret weapon known as the “Gravity Gun.” But before long, the Combine raid the safehouse, and the player must make a daring escape from The Rat Nest with their newly-acquired weapon.


The design of this level can be abstracted as three separate combat encounters, each having higher difficulty and tension than the last. Before the final encounter, there is a period without fighting while the player is introduced to the gravity gun. Not only does this reward the player for passing the first two combat encounters by giving them a break and a new weapon, it allows them just enough time to learn the basics of the weapon so that they can use it in the final combat encounter. After the final combat encounter, the level ends with a dramatic scripted event of a helicopter attacking the player.

Combat Encounters

I found that I could introduce a lot of replayability and improve the experience of combat encounters by giving the player multiple ways to dispatch enemies and letting them choose which path to take. I tried to build spaces that always allowed the player more than one viable path to follow wherever there was combat, and I considered each different weapon that the player has available in the level and ensured that the combat encounter would be fun to play with each weapon.

In the first encounter at the front gate, the player is shown several paths, and because the Combine do not attack on sight, the player can consider each path carefully and initiate combat when they are ready. They can charge straight at the gate, using wrecked cars as cover. They can climb the scaffold and throw grenades or shoot from above. They can hang back, sniping the combine with the revolver or drawing them out of their fortifications. Lastly, the player has the option to avoid the fight altogether, and climb across the rooftops in the alleyway to reach a vent and bypass the gate.

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The second encounter in the garage/market plays differently depending on if the player enters through the gate or sneaks through the vent. When entering through the gate, the Combine are strung across the balcony in a hasty ambush, presumably having heard the gunfire of the previous encounter outside the gate. Immediately the player comes under fire, but they have several options: to their left, right, and front are all areas of relative cover, and each of these areas can be passed through to move further into the room if the player wishes to advance. The enemies in this encounter are scripted specifically so that they will not move from floor to floor. This encourages the players to think about threats more vertically, since each floor will have enemies until the player moves through it and defeats them. If players entered through the vent, they are rewarded. They have the element of surprise, and the Combine will not fire until the player reveals himself by shooting or exiting the vent. Exiting the vent puts players on balcony overlooking the room’s center, giving them a height advantage over the enemies below.

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The third encounter encourages players to use the gravity gun they just received, but it is not required of them. The enemy spawns in this apartment area are scripted such that enemies will spawn in the rooms which the player does NOT enter first. This means that the room the player runs into will be safe, but the others will contain enemies. I found that spawning the Combine in this way encouraged a more mobile fight, as players would have to run from room to room in order to engage the Combine.

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Exploration Rewards

There are many optional areas and rewards for players to find throughout the level. This encourages exploration, and helps to break up the combat.Some rewards are simply ammo and health packs hidden in secluded areas:

  • Grenades and ammo on the scaffolding overlooking the main gate
  • Revolver ammo hidden on a table among junk items

Others are simple puzzles, in which a player is shown an area with a reward, but without an apparent way to reach it:

  • In the back of the Garage, a health and armor dispenser can only be reached by stacking crates to get over a chain link fence
  • An apartment bathroom containing a powerful rifle has a door locked from the inside. The player must drop through a vent to enter it

Finally, there is a more formal, complex puzzle in the apartment areas. A locked box with a small window contains health and ammo. Beside it are three terminals, one of which has a battery connected. The player must find two other batteries around the apartment, carry them to the box, and place them in the terminals. When all three batteries are in place, the box will unlock.

Puzzles like these reward players for thorough exploration of the environment. They appeal to and are rewarding to some players, but since they are entirely optional, players who ignore them will not be at a great disadvantage.

Game Design Portfolio of Chris Johnson