fort

be like Fort Knox (humorous)

if a building or an area is like Fort Knox, it is very difficult to enter or leave it because it is so well protected. Our house is like Fort Knox with all these extra security locks.

See also: like

hold the fort (British, American & Australian, American)

to be left in charge of a situation or place while someone is away. Someone had to stay at home and hold the fort while my mother was out.

See also: hold