Fortifications are militaryconstructions designed for defensivewarfare. They have been used for many thousands of years, in a variety of increasingly complex designs. The practice of improving an area's defense is also known as "fortification". Many military installations are known as forts, although they are not always "fortified".
... deep-level stations have closed platforms:
Charing Cross tube station (for the Jubilee Line)
Holborn tube station (for the Aldwych branch of the Piccadilly ...