Forts are a much more powerful version of the Archer Tower. They have more health than a Tower of Light or Archer Tower, and can fire arrows from all sides, allowing them to hit multiple enemies simultaneously. Citadel Upgrades allows trebuchets to be constructed on top of the fort, greatly increasing its firepower.
The Fortress game type requires players to destroy a set number of the enemy team's forts in order to win. In that game type a fort is called a "Fortress" and has 15,000 health.