Tremulous is a free and open source cross-platform first-person shooter game 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. 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.