Tremulous is a free, open source game that blends a team based FPS with elements of an RTS. Players can choose from 2 unique races, aliens and humans. Players on both teams are able to build working structures in-game like an RTS. These structures provide many functions, the most important being spawning. The designated builders must ensure there are spawn structures or other players will not be able to rejoin the game after death. Other structures provide automated base defense (to some degree), healing functions and much more...

Player advancement is different depending on which team you are on. As a human, players are rewarded with credits for each alien kill. These credits may be used to purchase new weapons and upgrades from the "Armoury". The alien team advances quite differently. Upon killing a human foe, the alien is able to evolve into a new class. The more kills gained the more powerful the classes available.

The overall objective behind Tremulous is to eliminate the opposing team. This is achieved by not only killing the opposing players but also removing their ability to respawn by destroying their spawn structures.

Features:
# Flexible particle system - 99% of the in-game visual effects are configured using particle scripts.
# 16 buildable structures with in-game functions.
# Play as several alien classes with unique abilities.
# Customize your setup as a human and buy new weapons, armour and items.
# Scale the walls and ceilings as an alien waiting for an unsuspecting human.
# Realistic physics and motion - no bunny hopping or quick back peddling.
# Flexible map system - animated mapobjects, triggering, light flares, etc.
# Large weapons system - don't like the weapon you have? Sell it and buy a different one, dozens of options.