Fortress-class warships were some of the largest and most powerful vessels in the Forerunner navy. From bow to stern the ships were approximately 50 kilometers long and were 10 kilometers across at the widest point. A single fortress ship can carry thousands of support craft.
The hull is divided into three sections: the foremost section of the ship is a massive hemispherical structure 10 kilometers in diameter. The dome's hollow core serves as living space for hundreds of thousands of Forerunner Warrior-Servants. Below the dome is a midsection composed of layered platforms supporting landing bays and weapon mounts. The aft section of the ship is a long "tail" covered in gun emplacements and other forms of energy weaponry. At the time of the Forerunner-Flood war these ships were up to 100 km in length with far more destructive capabilities, a battle group of these were capable of destroying at least one of the original 12 Halo Installations.
Fortresses were capable of deploying hundreds, if not thousands, of orbital picket cruisers, weapon-ships, and fast attack runners.
- The design of the fortress bears many similarities to the design of High Charity.