A Sub-Capitol is any one of four facilities located on each Empire's home continents which is connected via a green lattice link to the capitol facility. Sub-Capitols play an important role in the capture of capitol bases as they are the only way to disable the facility's impeneatrable shield barrier, called a Force Dome. If two or more Sub-Capitols are held by the same empire who owns the capitol facility, the capitol will activate it's Force Dome barrier. Only by taking three sub-capitals (or taking two and allowing another invading empire to take the third) can you disable this shield. The only way to successfully hack the capitol facility is by owning two or more Sub-Capitols.