1) Detached, independent outwork 
A réduit is a fortified structure such as a citadel or a keep into which the defending troops can retreat when the outer defences are breached. The term is also used to describe an area of a country, which either through a ring of heavy fortifications or through enhancing through fortification the defences offered by natural features such as moun...
inner fortified retreat for a garrison
• (n.) A central or retired work within any other work.
(from the article `Switzerland`) ...the commander in chief of the Swiss army, General Henri Guisan, expressed a lasting spirit of military resistance; in addition, a fortress in the ... ...possible aggression by the encircling Axis powers, he devised and implemented a plan of defense that forsook border areas in order to hold a ... [...
Detached, independent outwork.
Ré`duit' noun [ French See Redoubt , noun ] (Fort.) A central or retired work within any other work.