Half Life 2 – Of Mice and Freeman

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Level Overview

Development Info

Role: Sole Developer
Engine: Source
Genre: First Person Shooter
Development Time: ~100 Hours
Playtime: 5-10 Minutes

Level Features

 Gravity Gun only level
 Electrocution mechanic
Plug puzzles/ Collection puzzles



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High Concept

Solve puzzles, kill zombies, and maneuver through environmental obstacles using only the Gravity Gun.

Level Summary

“Of Mice and Freeman” is a single player, stand-alone level that takes place in the Half-Life 2 universe. The mysterious G-Man has kidnapped Gordon Freeman and placed him in an experimental testing chamber. Armed with only the Gravity Gun, Freeman is instructed to find his way out. Players must use the environment to overcome obstacles and solve puzzles. A new mechanic, using batteries to electrocute enemies, is introduced early in the level. Batteries can be dropped in standing water to instantly kill anyone touching the water (including the player). Players must also collect batteries and place them in receptacles to open locked doors.

At the end of the level, Freeman finally confronts the G-Man, who explains that the entire scenario was a job interview process for his new project: Aperture Science.


Design Goals

  • • Create a Gravity Gun only level with a new mechanic (battery electrocution)
  • • Use plugs to create interesting puzzles
  • • Gate the player by creating a collection mechanic

The Gravity Gun

I wanted to challenge myself as a designer through constraints by making a level that relies solely on the Gravity Gun. When players can only use that weapon, the first thing they do is look at the environment for things to manipulate. Of course, I did not want people to just grab heavy things and shoot them at enemies. I wanted there to be elements of risk and strategy. Expanding on mechanics I had seen while playing the game, I decided on batteries, electrocution, and plugs as core gameplay elements for my level. I explain those in detail in the sections below.

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Batteries, Water, and Doors

The primary way players can wipe out enemies in this level is by electrocuting them. I first introduce this to the player near the beginning of the level. A zombie walks toward the player through a puddle of water that has a battery in it. As soon as the zombie touches the water, it dies, showing the player this new mechanic. In the first few instances of this mechanic, the player is not likely to electrocute themselves. I wanted to give a safe space to experiment and understand how it works before I really test the player. Halfway through the level, the player faces scenarios where they must carefully navigate over large bodies of water without electrocuting themselves, adding to the danger of the level.


The entire point of batteries in this level is to bring them to receptacles that open locked doors. The level has two doors that require two batteries each to open. I liked this mechanic because it gave the player a long-term goal, and also set up scenarios to obtain the batteries. It gave a reason to carry batteries over great distances while also changing the gameplay to and from the location of the battery. Once the gameplay of obtaining the battery has been solved, returning the battery to its proper place is a completely different challenge (namely, how do I get this battery over this large body of water without killing myself?) I thought it was interesting that parts of the level that were benign before suddenly become dangerous. Because of this, I’m able to reuse gameplay space while making it feel different.

Plug Puzzles

Besides shooting objects, the Gravity Gun can also be used to manipulate plugs. This is the very first mechanic the player does in the level. Players must unplug a nearby socket to lower a force field that blocks them. I quickly reinforce this element in the next room by requiring the player to plug a socket to open a door. I find that teaching the player something twice in short succession helps them remember it.


The next time the player uses this mechanic is through a giant plug puzzle. There are several barriers that must be opened by manipulating plugs in order to obtain a battery. When entering this area, the player immediately sees the goal because the barriers are transparent. I really like showing the player a goal and making them feel a sense of progression when completing a puzzle, which is why I chose transparent barriers (and also they look cooler than regular doors). The important part of designing this puzzle was visibility. I had to make sure the player knew exactly the effects of using certain plugs and if that is how they want to proceed. To make paths clear, I used sound cues and bright, orange lights, which contrast heavily with the green/blue lighting in the room. During testing, these efforts paid off, as this was the favorite part of the level for most people.

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Post Mortem

I think I succeeded in creating the gameplay I wanted, and the response from players was extremely positive. Everyone loved the plug puzzle, which was the biggest risk in designing the level. The mechanics were built off of things I had already experienced in the game itself, which made them easy to understand for new players. I think the gameplay could easily fit somewhere in the Half-Life 2 universe.

I had some trouble making the level look as good as I wanted it to, as brush detail could be painstaking, and you cannot scale meshes in Hammer. Things that I thought would be minor ended up taking a large amount of time, such as getting the electrocution mechanic to feel right. And God help you if you want to move a plug at all after you set it, as doing that will make it lock in place 10 feet above the socket.

The editor itself was extremely stable and easy to use. Build-times were short, the scripting logic was intuitive, and visgroups helped solve bugs rapidly.