Tremulous is a free, open source team-based FPS licensed under the GPL, and it uses the icculus.orgQuake IIIioquake3 source code distribution. Reminiscent of Natural Selection and the Quake 2 modification Gloom, the game mixes pure FPS with RTS elements, and matches take place between two opposing teams: humans and aliens. Rather than relying on pure action, you can build constructions and activate upgrades that are unlocked gradually while playing.
Each team is fundamentally different. Humans use guns while aliens are class-based and rely on speed and melee attacks. By killing foes, aliens earn frags that can be used to evolve to stronger mutations. Humans, on the other hand, either spawn with a weapon or a construction kit, and they earn credits by killing aliens. Those credits can be exchanged in the armoury for new equipment. When a team reaches a set amount of frags, a new stage of development is occupied. Aliens get access to advanced creatures with healing and construction abilities, while humans acquire new building options, better weapons and stronger armour. There are three stages in total.
Each race has important constructions that must be defended. For the aliens, the Overmind is vital. It controls the production of eggs and the upgrade abilities. Humans have a Reactor as a main power source. There are additional defensive constructions such as barricades, alien tubes and turrets, or hives, eggs and telenodes as extensions of the main structures. These have distinct abilities as well. In an advanced development stage, aliens can build them on walls or ceilings, and they slow down humans with creep, or spray acid when destroyed. The overall objective is to destroy the opposing team.
The game was originally developed as a total conversion for Quake III, but later, after the release of the 3D engine's source code, turned into a standalone product. It supports OpenAL with surround sound.