A Fort is an original building from the game, The Conquerors. It is used for getting juggernauts, humvees, jeeps, soldier transports, general or artillery which may involve researching. It can also be used for defense, as it fires lasers at the enemy like a Command Centrer or Headquarters would.
Mostly in mid-term games, when a truce has already been broken, most people may put Forts around their base to get any outsiders from getting a Command Center or a priority item. Sometimes in the beginning of a game, people may use them to get defense from somebody who already has an army of soldiers. It has a garrison feature as well.
Due to Forts not being able to move, it is a good idea to put it around a priority item to be able to garrison subjects, defend a building, or produce subjects to stop enemy units. It should also be noted that four juggernauts can take down a fort quickly. Enemies will likely try to aim for the defensive building, such as Command Centers, HQs, and Forts. To avoid this, try to surround the building with turrets or walls, something with not much value.
Juggernaut: $65 (must be researched)
Artillery: $115 (must be researched)
Medi truck: $90
Humvee: $85 (must be researched)