From Halopedia, the Halo wiki
The stronghold is a pre-built structure that is dropped from orbit onto the surface of a planet, Halo installation, or other celestial body. Upon impacting the ground, massive spikes on the underside of the structure to burrow deep into the crust and act as a sturdy foundation.
After the UNSC's defeat at Installation 07 circa December 13, 2559, the Banished forces led by Escharum occupied the installation. These strongholds were deployed across the ring to maintain control, and would later become obstacles for the UNSC forces led by John-117 to overcome as they fought to stop the Banished in the months that followed.
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