Invasion is the Team Fortress 2 pre-release design concept. It was a futuristic sci-fi game featuring a war between humans and aliens.
The average Invasion game consists of two teams - the Humans and the Aliens. Gameplay is largely based around the collection of "Resources." Resources are chunks that can be picked up by players, which are then added to the Resource bank of their team. There are various ways for players to acquire chunks of Resource, including killing enemies, destroying enemy objects, or collecting 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.
When the Half-Life 2 source tree was leaked in late 2003, two models of Team Fortress 2 Commando class were included, along with the Team Fortress 2 source code, which was fully compilable.
In the end, this build was completely scrapped and nothing in this version would appear in the final version of Team Fortress 2. One of the Developer Commentaries in the final version of Team Fortress 2 mention that they had made the game too complex. 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. This iteration was mentioned in an August 2007 interview with Gabe Newell by GameTrailers and a September 2010 interview with PC Gamer, in which he mentions "Invasion" as being the second phase of Team Fortress 2's development under Valve Software.
Like Team Fortress Classic, it had 9 classes in all modes and one class for escort mode.
A heavy class, equipped with a 3-rocket rocket launcher, 3 hand grenades and a shield. Similar to Soldier. Had "adrenaline rush" ability (bull rush with shield, similar to Chargin' Targe) accompanied by battle cry.
A class, similar to Engineer, equipped with a burst rifle (firing limpets) and a shield. Was to build sentry guns. Could gain technologies: increased sentry gun count (could build one sentry gun by default, would increase this number to 3), improved sentry AI, improved sentry sensors and rockets for sentry gun (by default sentries had plasma guns).
A class similar to Spy. Could silently assassinate enemy players, disguise, consume corpses (if gained the technology for corpse consuming) and had thermal vision, a bullet-firing and a limpet-firing weapon.
A combat medic. Weaponry included a burst rifle, 3 grenades and a shield. Could heal players and, with special technology, repair any buildings in a range of up to 512 units from himself. The medic's health regenerated at around 10HP per second.
An equivalent for Pyro, equipped with a flamethrower, a gas can and a shield.
A fast and unarmed class similar to Scout. Had 25% more speed than most of other classes. Could gain radar scanner technology.
A class with EMP grenades, a Shotgun and a shield. Could improve his EMP grenades by gaining technologies: faster recharging EMP, medium range EMP, longer lasting EMP effect and huge radius EMP. Had slightly higher speed than most of other classes and self-boost ability that cost 35 energy points and multiplied damage by 1.5 for 15 seconds.
A class wielding sniper rifle. Could gain camouflage technology.
A class without any known special abilities. Only stub code exists in the leak.
An equivalent for Civilian from Team Fortress Classic intended for alien escort mode. Had shield, 5 grenades and an unknown bullet-firing weapon.