Tremulous is a free and open source team-based first-person shooter with real-time strategy elements. The game features two opposing teams: humans and aliens. Each team must attack the enemy’s base and team members while defending their own base. Tremulous is a first-person shooter with elements of real time strategy. Each team must construct and defend a base, consisting of structures which aid the players in some way. Players respawn from the spawn, one such structure. During a match, players may attack the enemy, or maintain the base and construct new structures. Humans utilize various weapons, armor and other upgrades, while aliens may evolve into more powerful classes, each of which possesses unique abilities. The teams do not have access to all possible upgrades and buildings at the beginning of the game. Each team must reach a threshold of frags in order to advance to the next developmental stage. The point at which a stage change occurs varies dynamically depending on the number of players on each team. Each new stage brings more upgrades for the humans and more classes for the aliens.