Invasion is the unofficial name for the second build of Team Fortress 2. It has never been shown and the only proof of its existence is code and two player models from the 2003 Half-Life 2 leak.
This build of Team Fortress 2 would be based on collecting an object known as Resource. Resources could be obtained by killing enemies, destroying enemy buildables or by getting them from a Resource Pump. When collected, Resources would be added to the team's Resource Bank, which would be used to create and upgrade buildings.
There would be a heavy emphasis on buildable objects. All classes would be able to build at least one object.
Teams could also upgrade their buildings by investing Resources into creating upgrades. Some upgrades include Self-Heal, which automatically repairs a damaged building, and Vehicle Boost, which makes a vehicle slightly faster.
Vehicles were to play a part in the game. A special building called the Vehicle Bay could be built, which would then be able to build vehicles. However, the building of these vehicles would need resources.
It is unknown how a team would be able to win. Most likely, a team would have to either gather a certain amount of Resources and/or destroy the enemy's Resource Bank.
The setting for the game would be Humans vs. Aliens. However, we do not know what the actual maps looked like, as the only art resources for the game known are two player models. Also, this build did not take place in the Half-Life 2 universe, despite rumors stating otherwise.
Based on the importance of buildings and collecting Resources, it seems as if this build would essentially be a real-time strategy game, but with classes (based on the Team Fortress classes) that are directly controlled by the player from a first person view.
All of the classic classes were to appear in the game in one form or another. There was also a new class added to the game.
Not implanted and has a small amount of code. Is based on the classic Scout.
Similar to his classic counterpart.
This is the only class that a model exists for. Is based on the classic Soldier.
This class is based on the classic Heavy. It would have weapons that would be very useful against anyone trying to destroy a building.
The Medic would be similar to his classic counterpart, but he could also repair builidings.
Support is based on the classic Demoman. It has only a small amount of code.
Similar to his classic counterpart.
The only thing that has been coded for this class is backstabbing.
Assumingly, this class would be the only class that could build the most powerful buildings.
The Escort is a new class that has a projectile shield.
This weapon fires a small burst when the fire key is pressed, simliar to the U.S. military's M16A2.
An automatic rifle.
Assumingly, this would be an upgraded Laser Rifle only available to certain classes.
This weapon would create a protective shield.
This weapon would repair damaged buildings.
Buildings would play a key part in gameplay. They would help protect a team's Resource Bank and provide offensive capabilities.
The Powerpack would be used to create power for buildings that needed power.
This weapon would be very similar to the classic Sentry Gun. It is unknown if it had multiple levels.
The Buff Station is similar to the classic Dispenser, in that it gives ammo and armor. It might also give the player health.
Barbed Wire would be used to deny enemies access to certain areas. It could be destroyed, but not without some effort.
A Bunker would be used as a base for defenders.
The Vehicle Bay would use Resources to build vehicles. Vehicles will be covered in the next section.
Surprisingly, this version of TF2 was to have drivable vehicles.
The Wagon would be the most basic vehicle. It could only be used to transport people between two points.
This vehicle would be used to take down certain defensive structures, such as the Barbed Wire.
The vehicle would be used if the player needed an offensive vehicle stronger and more durable than a Battering Ram.
This is essentially the Teleporter from Team Fortress Classic.
The Strider from Half-Life 2 would be a vehicle. Based on its performance in the Half-Life 2 games, it would be very deadly.
This would be a smaller, but cheaper version of the Strider.
This build of Team Fortress 2 was completely scrapped. Nothing in this version would appear in the final version of Team Fortress 2. Instead, the team took a completely different direction when they began making the next build; instead of trying to make a large, RTS-like FPS game, they went back and streamlined the classic Team Fortress games for a new generation of gamers, then added a unique art style.
It seems possible that this build's complexity is what killed it. One of the Developer Commentaries in the final Team Fortress 2 mention that they had made the game too complex, so as the make the Commander option from the first build useful. From what has been seen, the Brotherhood of Arms design was not very complex, but Invasion was. Its possible that Robin Walker, the dev that made the commentary, mixed up Brotherhood of Arms and Invasion.