Forerunner fort
From Halopedia, the Halo wiki
Forerunner Forts are Forerunner outposts guarded by large beam turrets and Sentinels. It also is surrounded by shield walls with ends meeting at the turrets. In Halo Wars, they are capable of retracting in on itself after sustaining enough damage. Twenty seconds after the fort retracts, it is available as a base location.
