Missile silo
From Halopedia, the Halo wiki
The UNSC Missile silo is a building in the game Halo Wars. According to the Halo Wars preview, most UNSC bases have at least one, sometimes more. The warhead looks like it's either nuclear or very large conventinal. It is likely that the missile is a "superweapon" for the UNSC, as "superweapons" are found in many RTS games.
